This message (We can’t connect to HMRC right now) usually appears when the information we used is not authorised to use. With that, make sure to enter the correct VAT registration number in the settings and login credentials. You can use this article as your guide: Take action to complete your Making Tax Digital (MTD) submission.
If you verified that the information you entered is all correct, we can perform some troubleshooting steps to isolate why you're unable to connect to HMRC. Let's access your QuickBooks account using an incognito window. Follow the shortcut keys below:
Google Chrome: Ctrl + Shift + N
Microsoft Edge: Ctrl + Shift + P
Firefox: Ctrl + Shift + P
Safari: Command + Shift + N
If it works, go back to your main browser and clear the cache. A piled cache can also be the reason for the unexpected issues on the pages we previously opened. If it persists, you can use other compatible browsers.
I've added this article on how to connect to HMRC to submit VAT: