It seems I solved the currency error (no idea if it is the right way, but no error for that now) But then I got another error since before closing an opportunity we have to add a Delivery Contact under Contact Roles on the opportunity. Think I solved that too (no error message for that now).

But we also have to add a territory before closing. I have no idea how to get that into the test class. Anyone who can help? This is my test class code now.

@IsTest (seealldata=true)
public class testCreateAssetonClosedWon{
    public static testmethod void testCreateAssetonClosedWon(){
    
       PricebookEntry pbID = [select Id,CurrencyIsoCode from PricebookEntry limit 1];
             
       //create a dummy account
        Account a = new Account ();
        a.Name = 'Test Asset Account';
        insert a;
        
        //create a dummy opportunity
        Opportunity opp = new Opportunity();
        opp.AccountId = a.Id;
        opp.CurrencyIsoCode = pbID.CurrencyIsoCode;
        opp.RecordTypeId = '012200000004fxZ';
        opp.Name = 'test Oppty';
        opp.StageName = 'A1 - Initiate';
        opp.CloseDate = system.today();
        insert opp;
        
        //System.Debug(pbID.CurrencyIsoCode);
              
        //create products
        OpportunityLineItem oli = new OpportunityLineItem();
        oli.OpportunityID = opp.Id;
        oli.PriceBookEntryID = [select id, CurrencyIsoCode from PriceBookEntry WHERE IsActive = TRUE limit 1][0].ID;
        oli.Converted_to_Asset__c = false;
        oli.UnitPrice = 10.00;
        insert oli;
        
        //create contact
        Contact newContact = new Contact(Lastname='testLastName', AccountId = a.Id);
        insert newContact;
        
        //create delivery contact 
        OpportunityContactRole ocr = new OpportunityContactRole (OpportunityId = opp.Id, ContactId = newContact.Id, Role = 'Delivery Contact');
        insert ocr;
        
        Test.startTest();
            opp.StageName = 'A6 - Closed Won';
            update opp;
        Test.stopTest();
    }
}

Also please let me know if it does not do what it is supposed to now. I have no idea what I am doing...

I am guessing I have to make the opportunity close since that is when the products get automatically turned into assets on the account page? But I have never had to add a territory to a test class before which makes me question if this really is the correct way to do it.