Hello there, Richard Paterson.

Appreciate you for letting me know that you've tried following some possible solutions to resolve the same error you've got. Still, I just want to include possible causes why that error appear.

• You have not registered for MTD.
•  You've registered for MTD but aren't connected to HMRC.
• In QuickBooks, you must re-authenticate MTD.
• You entered the wrong credentials for the Government Gateway.
• Make sure you're using the right VAT number.
• An accountant set up the MTD connection.
• A client established the MTD connection (for accountants)
• You have no outstanding HMRC commitments.

For solutions, please check this article: Take action status when connecting to HMRC for Making Tax Digital (MTD). 
 

If you're sure about the setup, can perform some troubleshooting steps to fix this. Let's start by accessing your account using a private browser. There are times that the browser is full of frequently accessed page resources, thus causing some errors and/or unusual responses. You can use either of these keyboard shortcuts:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + P
• Safari: Command + Shift + N

If it works successfully, go back to your regular browser and clear the cache to prevent any unexpected behavior. Otherwise, you can use other compatible browsers in QBO. 

If the same thing happens, please reach out to our customer support. They  would be really helpful because they have the capacity to check into your personal account in-depth, see what's going on, and even screen share with you if necessary. You can utilize the Help icon within QBO.

Visit again if you have more concerns or need still need assistance with this matter. Take care and stay safe, Richard Paterson.