Hi Friends,
Can any one please help for this error?.
I have to design a custom button for converting records in opportunity.
I have a table in which I am displaying data from Account, contact and 3 custom objects like attached pic. Whenever I'll select any record from this table using checkbox and click on that button then it should create opportunity. 
I'm unable to get data of related object records like Account name, Contact name.
My code is as below:
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 global static string ConvertToOpportunity(Integer dataId){    
                
        Recommendation__c rec = [Select name, Account__r.Id, Account__r.Name, Recommendation_Contact__r.Name,Recommendation_Contact__r.Contact_YTD_Sales__c, Feedback__c,                                       Account__r.AccountNumber ,Recommendation_Contact__r.Title , Product_Service__c, Type__c, Opportunity_Score__c, Next_Best_Action__c, Product_Recommendation__c, Primary_SKU__c,  Recom_External_Id__c from Recommendation__c where Recom_External_Id__c =:dataId];
        system.debug('Recommendation:' +rec);
        
        if(rec != null){
        Opportunity opp = new Opportunity();
            opp.Name = rec.name;
            opp.Account_ID__c = rec.Account__r.Id;
            //opp.Account = rec.Account__r.Name;
            //opp.Account = String.valueOf(rec.Account__r.Name);
            // opp.Contact__c = rec.Recommendation_Contact__r.Name;
            opp.Recommendation_Name__c = rec.Name;
            opp.Opportunity_Lead_Score__c = rec.Opportunity_Score__c;
            opp.Contact_YTD_Sales__c = rec.Recommendation_Contact__r.Contact_YTD_Sales__c;
            opp.Conversation_Type__c = rec.Type__c;
            opp.Product_Recommendation__c = rec.Product_Recommendation__c;
            opp.Next_Best_Action__c = rec.Next_Best_Action__c;
            opp.CloseDate = system.today();
            opp.StageName = 'Pre-Qualified';
            
            system.debug('Opportunity:' +opp);
        insert opp;
        }
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