[SalesForce] SObject row does not allow errors

This error arises when you try to call addError in a trigger but call it on a record that is not in the trigger collection, which is exactly what you are doing. You can only validate records that are being acted on.

The code you have shared will not at all compile.

• use close brace here for (Opportunity childObj : Trigger.new)
• use childObj.AccountId instead of childObj.Account to access AccountId
• When you use relationship query be sure to use Opportunities instead of Opportunity.
• You should put Opportunity's Opp_Location__c into a List. Then use String.join() to add those with comma delimited.
• And finally update accounts.

Trigger

trigger UpdateAmount on Opportunity (after insert, after update) { 
  Map<ID, Account > parentOpps = new Map<ID, Account>(); 
  Set<Id> listIds = new Set<Id>();

  for (Opportunity childObj : Trigger.new) {
    listIds.add(childObj.AccountId);
  }

  parentOpps = new Map<Id, Account>([SELECT id, Location_Type__c ,
                                        (SELECT ID, Opp_Location__c  FROM Opportunities) 
                                    FROM Account WHERE ID IN :listIds]);

  List<Account> lstAccount = new List<Account>();

  for(Account acct:parentOpps.values())
  {  
      List<String> strType = new List<String>();
      for(Opportunity oppty:acct.Opportunities)
      {
          strType.add(oppty.Opp_Location__c);
      }
      acct.Location_Type__c = String.join(strType,',');

  }

  update parentOpps.values();
}