[SalesForce] FIELD_INTEGRITY_EXCEPTION when creating Price Book Entry from flow

Like I said in comment your Opportunity.Pricebook2Id should be equal to your OLI.pricebookEntry.Pricebook2Id

So for solution what you can do..

Query only those PricebookEntry where pricebookEntry.Pricebook2Id = Opportunity.Pricebook2Id

change your query in constructor and search method..

Note: Please be insure your opportunity pricebook2Id not null

public with sharing class pageclass {

public GEN_ProductSelectionController(){

    wrapProductList = new List<wrapProduct>();
    Id objPricebook2Id = [SELECT Pricebook2Id FROM Opportunity 
                           WHERE Id =: objCurrentOppId]; 
    for(PricebookEntry a: [select Id, Name,ProductCode,UnitPrice,product2.Type__c 
                            from PricebookEntry WHERE PriceBook2Id =: objPricebook2Id]) {
      wrapProductList.add(new wrapProduct(a));
    }
    System.debug('Default product list :' +wrapProductList);
}

Also in your search method change PricebookEntry query and add filter PriceBook2Id =: objPricebook2Id

You most likely have a Process Builder or a trigger, or wfr setting the price book on insert of the opportunity.

You did debug and that is good but you only debugged what you created not the results after it was committed to the database.

Before your debug, query for the opportunity and check if the pricebook2id is the same and what your set it to be:

Opportunity tmp = [Select PriceBook2Id from Opportunity Where Id = :op.id];
system.assertEquals(pricebookId,tmp.PriceBook2Id,'The Opportunity price book was modified after it was inserted');