Will not let me pay my VAT by MTD and hav used it for a long time

I'd love to help you here in the Community, Dg2. 

To do more digging into this, Did you encounter an error message while trying to pay your VAT by MTD? Or can you please provide me with more details or screenshots? This way, I can outline the best possible resolution for you.   

Just in case, I'll add this article for future reference: Everything you need to know about Making Tax Digital (MTD) for VAT with QuickBooks Online.

I'll keep an eye open for your response. Have a great day!