Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Have you already set up Microsoft Authenticator for your Microsoft 365 admin account (the account created when first purchased the business subscription)?
Please open the Microsoft Authenticator app on your phone and check whether you can see your Microsoft 365 Admin account (the admin email address you use to sign in to the Admin Center).
- For work account, it will show 6 digit code.
- For personal account, it will shoe 8 digit code.
So if you are seeing 8 digit code, it is not the admin account for your work.
If the admin account is not listed in the Authenticator app, the MFA setup may not have been completed. In this case, can you still log in via https://mysignins.microsoft.com/security-info with your admin credentials.
And also, this is user to user forum, if you'd like to have an support engineer to work with you, you will need to open a support ticket. But before doing that, please let me know your situation and I will help you with the next steps.