I'm thrilled that you posted here in the Community, mark.
Let me help you with this. As per checking submitting a VAT on the end date of the month will return you and show that it's still in process. The reason why you're getting an error.
I'll recommend you submit it again today and check it after.
You can read this article if you still encounter the same issue: Fix MTD issues and errors in QuickBooks Online.
If you have further questions about the error in submitting VAT, don't hesitate to respond again. Have a good one!
