[SalesForce] Create a New Record with lookup values

The trigger code is having lot of redundancy, So I modified it. before posting the modified code few lines of explanation what I changed. you collected account Ids twice and did the condition check twice where once is more enough. I am also made changes to the test class, where Task data is provided which is not required because the trigger has to create it once the necessary conditions are met. I am expecting this trigger should work for you, if there is any errors feel free to post as a comment.

trigger OpportunityAfter on Opportunity (after update){

    Set AccountIDs = new Set();

    for(Opportunity o : trigger.new){
        if(opp.StageName == 'Closed - Lost' && opp.Business_Line__c == 'Workforce' && 
            opp.Survey_Type__c != 'APulse' && trigger.oldMap.get(opp.Id).StageName != opp.StageName){

                AccountIDs.add(o.AccountId);
         }
    }

    Task[] tasksToInsert = new Task[]{};

    for (Account acc : [Select Id, Name, OwnerId from Account where Id IN :AccountIds]){
        for (Opportunity opp : trigger.new){
            if(opp.AccountId == acc.Id){
                Task tas = new Task();
                tas.WhatId=acc.Id;
                tas.OwnerId=opp.OwnerId;
                tas.Subject='Follow-up: ' + opp.name + ' - ' + acc.name;
                tas.ActivityDate=(opp.CloseDate + 180);
                tas.Description='Follow up to see how engagement initiatives are going and if Avatar can help. Refer to lost opportunity: ' + opp.name;
                tasksToInsert.add(tas);                                                         
            }
        }
    }
    if(tasksToInsert != null && !tasksToInsert.isEmpty())
    Database.insert(tasksToInsert);

}

@isTest
public class TestClass {

    static testMethod void myUnitTest() {

    //Set up user
    User u1 = [SELECT Id FROM User WHERE Email='abc@sampleemail.com'];
    test.startTest();  
    //Run As U1
    System.RunAs(u1){

    System.debug('Testing trigger... (single record validation)');

    //NEW Account record
    Account acc = new account();
    acc.Name = 'Test Account';
    acc.Industry = 'Healthcare';

    insert acc;

    //NEW Opportunity record
    Opportunity opp = new Opportunity();
    opp.Name = 'Test Opportunity'; 
    opp.CloseDate = System.today();
    opp.StageName = 'S1 - Investigative';
    opp.Type = 'Repeat Business - Wrkforce HC';
    opp.AccountId = acc.Id;
    opp.Survey_Type__c = 'Product Name';

    insert opp;

    opp.StageName = 'Closed - Lost';
    opp.Reason_Won_Lost__c = 'Relationship';
    opp.Business_Line__c = 'Workforce'  //added this line to meet the if condition while collecting the set of id in the trigger.

    update opp;
   test.stopTest();
    // Here I am expecting the task has been inserted and doing a query for it         and check it in the assert statement.
   Task  testTask=[Select Id, WhatId from Task where WhatId=:acc.Id];
    //Validate single insert
    System.assertEquals(acc.Id, testTask.WhatId);

    }
    }
}

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();
}