[SalesForce] Unable to see App Launcher navigation item or App Launcher inside App Launcher Lightning App

This is answer to my own Question.

Permission "Enable Salesforce Identity", needs to be enabled by Salesforce Support. My Organization was not having this permission.

Now, its Working.

You can't generally just put statements wherever you want to. As an exception, you can declare an initial value when you declare the variable, but only if you don't use get/set. Usually, we use the "constructor" for initializing the page state. Your code should work as follows:

public  class stageDocus
{
    public String currentRecordId {get;set;}
    public Opportunity Oppo{get;set;}
    public stageDocus() // Default constructor
    {
        currentRecordId  =  this. ApexPages.currentPage().getParameters().get('Id'); 
        Oppo = [SELECT Id, Name, StageName, Amount FROM Opportunity WHERE Id = :currentRecordId];
    }
    public List<Document> getdocs()
    {
            List<Document> doc = [SELECT Id, Name, FolderId, Folder.Name FROM Document WHERE Folder.Name like :Oppo.StageName];
            return doc;
    }
}

However, I strongly recommend against this design. It's usually more practical to use an Extension instead. Your page's first couple lines would change as follows:

<apex:page standardController="Opportunity" extensions="stageDocus">
    <apex:outputText value="{!Opportunity.StageName}" rendered="false" />

The second line causes Visualforce to automatically include the StageName field in the Visualforce's automatic query.

Your class would then change as follows:

public  class stageDocus
{
    ApexPages.StandardController ctrl;
    Opportunity Oppo;

    public stageDocus(ApexPages.StandardController c) // Extension constructor
    {
        ctrl = c;
        Oppo = (Opportunity)c.getRecord();
    }
    public List<Document> getdocs()
    {
            List<Document> doc = [SELECT Id, Name, FolderId, Folder.Name FROM Document WHERE Folder.Name like :Oppo.StageName];
            return doc;
    }
}