This error when submitting VAT return is often caused by outdated browser. We can try a few troubleshooting steps to resolve this. Let's start by using incognito browser to rule out the possibility of a web page issue. Private browsing doesn't store local files or cache.
Chrome, press Ctrl + Shift + N.
Mozilla Firefox: press CTRL + Shift + P
Safari, hold down Command + Shift + N.
If this removes the error, clear the cache of your regular browser to help QBO function more efficiently. Otherwise, you can use supported browser as alternatives.
If all else fails, I recommend contacting the HRMC. They can help review your user ID to make sure you can submit your tax return.