The bind expression, opp.Account, in your code, refers to the Account SObject. Not the Account Name.

 I refactored some of the code to show you how you need to query all the related Accounts, can store them in a Map for easy access, and then iterate across the Opportunities. I hope this helps.

trigger getRelatiecode on Opportunity (after insert) { System.debug('Making future call to sfservice'); List<id> accountIds = new List<Id>{}; String UID=UserInfo.getUserId(); String UPW=UserInfo.getFirstName(); for (Opportunity opp : Trigger.New) { if(opp.AccountId!=null) accountIds.add(opp.Account); } Map<Id,Account> accountMap = new Map<Id,Account>([select id,MarktSelect_Sleutel__c, name from account where Id IN :accountIds]); for(Opportunity opp: Trigger.new){ //Call future method to update account //with data from external server. //This is a async calls, it returns right away, after //enqueuing the request. //string MK = [SELECT MarktSelect_Sleutel__c FROM Account WHERE Name = :smileysurprised:pp.Account]; Account MK = accountMap.get(opp.AccountId); integer MKR = integer.valueof(MK.MarktSelect_Sleutel__c); //SFservice.sFservice(opp.id, UID, UPW, MKR); } }