Hey there, @y_dunlop.
I've got some ways to provide for you to be able to create an agent services account for HMRC's MTD services.
Firstly, ensure to use the accurate credentials provided by HMRC, especially for MTD the moment you set up MTD in QuickBooks.
Moreover, if you have accidentally used credentials that aren't MTD, you have to disconnect and reconnect to MTD using the accurate ones.
However, it's important to keep in mind to avoid using any other credentials as this can cause authentication issues which explains why you've experienced those.
Furthermore, here's an article you can browse in case you need assistance submitting your VAT return to HMRC using MTD: Submit a VAT return to HMRC using MTD in QuickBooks Online.
You can always get back to us at any time, know that we'd be glad to assist 24/7. Have a good one.
