[SalesForce] Error Using Flows – Required Field Missing

You need to set the Account on the new Account_Detail__c records. Try do something like this:

for(Account a : loops over a certain amount of times) {
    Account_Detail__c j = new Account_Detail__c();
    j.name ='blank';
    j.AccountId = a.Id;
    detailList.add(j);
}

You have to make the Custom Button as an onclick javascript button and then open the VF Page by passing the opportunity id as a parameter to the page (say call id oppId) and then pass that parameter to the flow..

so your Custom Button properties will be

• Display Type - List Button
• Behaviour - Execute Javascript
• Content Source - Onclick Javascript

and then in the script section point it to your VF page with the param

window.location = '/apex/YourVFPage?oppId={!Opportunity.Id}'

then in your VF page you can do

UPDATE :

the param tag was outside the flow tag which is why the value was not getting passed into the flow.. please use the correct syntax as below

<apex:page standardController="Currency_Detail__c" recordSetVar="Currency_Details">
   <flow:interview name="Create_Currency_Detail_Records" finishLocation="/{!Id}">
       <apex:param name="OppId" value="{!$CurrentPage.parameters.oppId}"/>
   </flow:interview>
</apex:page>