Thank you for providing the specific error, VRNI. Let me share some insights regarding this.
The QuickBooks Online (QBO) error CLIENT_OR_AGENT_NOT_AUTHORIZED commonly appears when you need to reauthorize the connection between QuickBooks Online and HMRC, when you set up Making Tax Digital (MTD) and the Value-Added Tax (VAT) registration number in QuickBooks does not match HMRC's records, or when you haven't entered the correct credentials for MTD provided by HMRC.
Since you've mentioned that you can't connect to HMRC yet the VAT number is correct, it's best to check the email confirmation from HMRC. This email will confirm whether or not your connection has been successful. Please know that this can take up to 72 hours. This will have your new MTD credentials which you'll need to set up the MTD in QuickBooks.
I'll leave you with some resources to use while setting your MTD and to learn more about the reasons why this error happens:
Furthermore, you can also submit a VAT return to HMRC using MTD in QBO.
Feel free to leave a comment if you have further questions or information and we'll be right there to assist you.
