I can’t submit my VAT return It says: taxReturninvalidMTDEndDate

Thanks for posting to the Community, @short9jh.

This error means, you're unable to submit this return today as the quarter is not closed yet. You can contact our Customer Care Technical Support. They have the advanced tools to pull-up your account and to verify when is the right submission date of your VAT return.

Here's how to reach support:

1. Go to Help and then select Contact Us.
2. Please enter your query and then click Let's Talk.

You can refer to this article for more details: Submit a VAT return n QuickBooks Online. It includes there the reasons for unsuccessful submissions.

Please get back through this post after you reached the support if you need more assistance. I'm always here to help. Have a good one!