This error message typically displays when the login credentials used to connect QuickBooks to HMRC are not authorized to retrieve information or submit a VAT return for the VAT registration number used in the VAT settings.
Thus, ensure first that you signed up for MTD or turn on MTD in QuickBooks and connect it to HMRC. This usually arrives within 22 hours to 72 hours. If so, the next thing you'll need to do is to connect your QuickBooks Online (QBO) account with HMRC's MTD service using your HMRC credentials.
Afterward, HMRC will confirm if it's successful or not by sending a message to your email. So, it's advisable to see your email from time to time. The said email includes your MTD credentials. If you haven't received it, I suggest contacting HMRC. This typically arrives between 22 to 72 hours. If this is the case, the next step is to use your HMRC credentials to connect your QuickBooks Online (QBO) account to HMRC's MTD service. It's also best to check your email messages from time to time since it'll send an email confirmation and it also includes your MTD credentials.