I can see that you did everything you could, @AndrewBoff. I'm here to ensure you'll be able to get rid of the error and submit your VAT returns.
Let me first discuss some details about the process.
When submitting a VAT return in QuickBooks Online (QBO), you have to ensure that you're using the correct Government Gateway user ID and password given by HMRC. Since you've mentioned that you've just moved over to MTD, there should be an email notification confirming that you can start using the account and submit the client's return. For reference, please check this article: How to fix MTD issues and errors in QuickBooks Online.
If in case you did not get any email confirmation, I recommend checking your inbox or junk folder. Then, look for the following:
If you still can’t, please contact the HMRC. This way, you can verify your details are correct. Feel free to check this link for more info: What should I do if I didn't receive a confirmation email?.
Keep me posted again here if you have any other concerns about submitting your VAT return. I'm just around the corner to help. Have a good one and take care.
