I'm having an issue with my Apex Test Class from a trigger. I've used the trigger multiple times in my sandbox but when I try to test it out it fails. The reason for the failure is 'System.QueryException: List has no rows for assignment to SObject' which I have tried to look up but there are so many answers and they don't apply to my certain case. Tried it multiple ways and I am wondering if anybody here has a good answer.

I have two objects, Implementation (custom) and Opportunity. The trigger fires when an Opportunity is set to 'Closed Won'. Below is the trigger:

trigger CreateImplementation on Opportunity (after update) {

    List<Id> opps = new List<Id>();

    for ( Opportunity opp : Trigger.new ){


        if((opp.StageName == 'Closed Won' ) &&
        (Trigger.oldMap.get(opp.Id).StageName!='Closed Won') ) {

        opps.add(opp.Id);

    }

    Map<Id,Opportunity> oppsMap = new Map<Id,Opportunity>( [SELECT Id, Account.Id, CloseDate, OwnerId, Account.Name from Opportunity where Id IN :opps] );

    List <Implementation__c> impToInsert = new List <Implementation__c> ();

    for( Opportunity oppo : oppsMap.values()){
        Implementation__c imp = new Implementation__c ();

        imp.Account__c = oppo.AccountId;
        imp.Name = oppo.Account.Name +' Implementation';
        imp.Phase__c = 'Phase 0';
        imp.Contract_Date__c = oppo.CloseDate;

        impToInsert.add(imp);
    }

try {
        insert impToInsert;
    } catch (system.Dmlexception e) {
        system.debug (e);
    }
    }
}

And here is the test: 

@istest private class CreateImplementation {
testMethod private static void CreateImplementation () {

 Account acco = new Account(Name = 'Acme Test Account', Type = 'Prospect');
    insert acco;

Opportunity opp = new Opportunity(Name = 'Acme Test Opp', CloseDate=System.Today(), AccountId=acco.id, Type= 'New Business', Business_Value__c='NA',Closed_Reasons__c='NA', StageName='Closed Won',Amount=1000);
    insert opp;

 Implementation__c imp = [ SELECT Account__c, Name, Phase__c FROM Implementation__c WHERE Account__c = :acco.id LIMIT 1 ];
 System.assertEquals(imp.Phase__c,'Phase 0');}}

Thanks in advance!