I cant complete my VAT return as there is an "undefined error". Is anyone else having this problem?

Welcome to the Community, @jgvenn.

I've checked it here and this was previously reported to our engineers. But don't worry, they've provided a resolution to help other users.

Firstly, you'll need to turn off Making Tax Digital (MTD) in your QuickBooks account. To achieve this, you can follow the steps below:

1. Click on Taxes from the left panel.
2. From the Edit VAT drop-down, choose Edit Settings.
3. Click on the Turn off MTD button.
4. Click on Save.

Once done, you can then mark the return as filed. But note that you don't need to e-file the return.

Finally, you'll need to turn on your MTD again and resubmit your VAT return.

But if the issue still persists, you can contact our QuickBooks Online Support team. This way, they can investigate this behavior further.

Here's how you can contact them:

1. Log in to your QuickBooks account.
2. Click on Help.
3. Choose Contact Us.

Don't hesitate to get back to me if you have any other concerns. I'll be here to help you.