I'm trying to complete a VAT return but get this message:taxReturnInvalidMTDEndDateMy Making Tax Digital is enabled

Hi @peter_fuller, thanks for posting your concern here. 

I'm here to help you complete your VAT return in QuickBooks Online (QBO) without getting an error.

If you received an error even after enabling MTD, there must be some other issues like browser-based, or cache documents that might be the cause of this error. To isolate this, let's do some troubleshooting steps.

I suggest logging in to your QuickBooks using an incognito browser to rule out the possibility of a webpage issue. Private browsing doesn't store local files or cache. You can use these keyboard shortcuts based on the browser you're using. 

• Mozilla Firefox or Microsoft Edge: CTRL + Shift + P
• Google Chrome: CTRL + Shift + N
• Safari: Command + Shift + N

Once done, please complete your VAT return again. If it works without issues, go back to your regular browser and clear the cache to fix the unexpected behavior in the product. If not, use other supported web engines. The current one you're using may have a temporary issue with QuickBooks. Utilizing a new browser allows you to get back to working order.

You can read this article if you still encounter different issues in the future: Fix MTD issues and errors in QuickBooks Online.

I'll be here in the Community space if you require further assistance in completing your VAT return. Stay safe, and have a good one.