These problems can occur for different reasons, one of the most common is when your email has been used by another person and this person had the Microsoft Authenticator application configured on their phone, so I recommend resetting the entire authentication process by following the following Steps :
Go to the Azure Active Directory page
Select the Users option
- Select the user who has the authentication problem
- Then select the Authentication Methods option in the menu on the left
- Select the "Require re-register multifactor authentication" option and restart the process
You can also try to check the "Switch to the new user authentication methods experience" option by clicking the arrow icon -> and then you can delete the registered MFA authentication methods for the selected user.
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