[SalesForce] Error: Unknown method in Visualforce

Could you help me to check my error?
I have problem when i save my visualforce,
I get Error like this :

Unknown method 'functionCountSeverence.RunSeverence()'

I was declare my class with : public with sharing class functionCountSeverence(){}

But, I still get the errorm. Here My Visualfprce page code :

<apex:page controller="functionCountSeverence">
    <apex:form >
        <apex:pageBlock title="Severence">
            <apex:pageBlockSection columns="1" showHeader="false" >
             <apex:commandButton id="RunPreProcess" value="Process Severence"
                                    onclick="return confirm('Are you sure to process this Severence ?');"
                                    action="{!RunSeverence}"
                                    />    
            </apex:pageBlockSection>               
        </apex:pageBlock>
    </apex:form>
</apex:page>

And This is my Class controller code :

public with sharing class functionCountSeverence {

    //Declare Variable:
    Public Date vPaymentStartDate;
    public Date vSalaryStartDate;
    public Date vSalaryEndDate;
    public Id   VFLDEmpElmHeaderId;
    public Id   VFLDEmpElmHeaderEmpNo;

    //MAP Declaration :
    Map<String,PYRESYTDHEADER__c> PYRESYTDHEADERMap = new    Map<String,PYRESYTDHEADER__c>();    

    public string RunSeverence(String selectedPayrollName, Boolean ProcessAll, SelectOption[] selectedEmployee, String selectedPackage)
    {
        string message;
        //===================================== Get Salary Start Date and Salary End Date from SETPERIOD ======================================
        //Query from Payroll Period to Get Record for Employee Element by Period
        PYSETPAYPERIOD__c[] VFLDQPayPeriod = [SELECT Salary_Start_Date__c, Salary_End_Date__c, Payment_Start_Date__c
                                              FROM PYSETPAYPERIOD__c WHERE Id=:selectedPayrollName]; 

        PYSETPAYPERIOD__c eSinglePayPeriod = new PYSETPAYPERIOD__c();
        for (PYSETPAYPERIOD__c VFLDLSTPayPeriod : VFLDQPayPeriod)
        {
            eSinglePayPeriod = new PYSETPAYPERIOD__c();
            vPaymentStartDate   = VFLDLSTPayPeriod.Payment_Start_Date__c;
            vSalaryStartDate    = VFLDLSTPayPeriod.Salary_Start_Date__c;
            vSalaryEndDate      = VFLDLSTPayPeriod.Salary_End_Date__c;
        }
        //=================================== End Get Salary Start Date and Salary End Date from SETPERIOD ====================================== 
        //-----PYEMPELEMENTHEADER__c (Employee Header)
        LIST <PYEMPELEMENTHEADER__c> VFLDQPYEMPELEMENTHEADER = [SELECT Assignment_No__c, Company_Name__c, Cost_Center_Code__c, Employee_No__c, 
                                                                Employee_No__r.Full_Name__c, Employee_No__r.Effective_Date__c, Grade_Name__c, 
                                                                Job_Title_Name__c, Organization_Name__c, Payment_Start_Date__c,Period__c, 
                                                                Period__r.Name, Position_Name__c, Package_Name__c, Tax_File_No__c,
                                                                Tax_Location_Name__c, Tax_Location_Name__r.UMP_Limit__c, Employee_No__r.Name, Tax_Status__c, 
                                                                Tax_Type__c, Work_Location_Name__c, Id, Employee_No__r.Full_Name__r.Gender__c, 
                                                                Employee_No__r.Full_Name__r.Marital_Status__c , Is_Severence_Pay__c
                                                                FROM PYEMPELEMENTHEADER__c 
                                                                WHERE Period__c =: selectedPayrollName
                                                                AND Package_Name__c =: selectedPackage AND Payment_Start_Date__c =: vPaymentStartDate 
                                                                ORDER BY Payment_Start_Date__c, Employee_No__c];

        //-----PYRESYTDHEADER__c (YTDH) Table 
            String vCurrentYear     = String.valueOf(vPaymentStartDate.year());
            LIST <PYRESYTDHEADER__c> VFLDLSTQPYRESYTDHEADER = [SELECT Employee_No__c FROM PYRESYTDHEADER__c
                                                                       WHERE Current_Year__c=:vCurrentYear];
        for(SelectOption VFLDLSTSlcEmpList : selectedEmployee)
        {
            Id  VFLDSelEmpVal   =   VFLDLSTSlcEmpList.getvalue();

            for (PYEMPELEMENTHEADER__c VFLDPYEMPELEMENTHEADERList : VFLDQPYEMPELEMENTHEADER)
            {
                VFLDEmpElmHeaderEmpNo   = VFLDPYEMPELEMENTHEADERList.Employee_No__c;                
                if(VFLDSelEmpVal == VFLDEmpElmHeaderEmpNo)
                {
                    String vYear            = String.valueOf(date.valueOf(VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c).Year());
                    String KeyId            =  vYear+ VFLDPYEMPELEMENTHEADERList.Employee_No__c;
                    Integer vCurrentPeriod  = VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c.Month()-VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c.Month() + 1;
                    PYRESYTDHEADERMap.put(KeyId, new 
                              PYRESYTDHEADER__c(
                                  Company_Name__c            = VFLDPYEMPELEMENTHEADERList.Company_Name__c,
                                  Cost_Center_Code__c        = VFLDPYEMPELEMENTHEADERList.Cost_Center_Code__c,
                                  Current_Month__c           = VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c.Month(),
                                  Current_Period__c          = vCurrentPeriod,
                                  Current_Year__c            = String.valueOf(VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c.Year()),
                                  Effective_Month__c         = VFLDPYEMPELEMENTHEADERList.Payment_Start_Date__c.Month(),
                                  Employee_Name__c           = VFLDPYEMPELEMENTHEADERList.Employee_No__r.Full_Name__c,
                                  Employee_No__c             = VFLDPYEMPELEMENTHEADERList.Employee_No__c,
                                  Gender__c                  = VFLDPYEMPELEMENTHEADERList.Employee_No__r.Full_Name__r.Gender__c,
                                  Grade_Name__c              = VFLDPYEMPELEMENTHEADERList.Grade_Name__c,
                                  Is_Severence_Pay__c        = VFLDPYEMPELEMENTHEADERList.Is_Severence_Pay__c,
                                  Job_Title_Name__c          = VFLDPYEMPELEMENTHEADERList.Job_Title_Name__c,
                                  Organization_Name__c       = VFLDPYEMPELEMENTHEADERList.Organization_Name__c,
                                  Position_Name__c           = VFLDPYEMPELEMENTHEADERList.Position_Name__c,
                                  Tax_File_No__c             = VFLDPYEMPELEMENTHEADERList.Tax_File_No__c,
                                  Tax_Location_Name__c       = VFLDPYEMPELEMENTHEADERList.Tax_Location_Name__c,
                                  Tax_Status__c              = VFLDPYEMPELEMENTHEADERList.Tax_Status__c,
                                  Tax_Type__c                = VFLDPYEMPELEMENTHEADERList.Tax_Type__c,
                                  Work_Location_Name__c      = VFLDPYEMPELEMENTHEADERList.Work_Location_Name__c

                                ));
                }
            }
        }
        //-------------------------------------Validation for Check Duplicate record on YTDH--------------------------------------------------        
                LIST <Id> VFLDSTYTDHDelId = new LIST<Id>();

                for(SelectOption VFLDLSTSlcEmpList : selectedEmployee)
                {
                    Id  VFLDSelEmpVal   =   VFLDLSTSlcEmpList.getvalue();
                    for(PYRESYTDHEADER__c VCompareYTDList : VFLDLSTQPYRESYTDHEADER)
                    {
                        if(VFLDSelEmpVal == VCompareYTDList.Employee_No__c)
                        {
                            String TempId = String.valueOf(VCompareYTDList.Id);
                            VFLDSTYTDHDelId.add(TempId);                            
                        }
                    }
                }

                LIST<PYRESYTDHEADER__c> VFLDYTDHDelete = [SELECT Id FROM PYRESYTDHEADER__c WHERE Id IN:VFLDSTYTDHDelId];

                Integer CountUpdEmpElm = VFLDYTDHDelete.size();

                if(CountUpdEmpElm != 0)
                {
                    delete VFLDYTDHDelete;
                }

                //Insert to PYEMPELEMENT__c table:
                insert PYRESYTDHEADERMap.values();
            //============================================ END PROCESS INSERT TO YTDH ===============================================                
        return message;
    }   

}