If the Microsoft Authenticator app is set as your primary authentication method and you have multiple authentication methods configured, but you are unable to use the app, please select the 'I can't use my Microsoft Authenticator app right now' option.
This will bring you to another screen where you can select the other authentication method(s) you have configured to successfully authenticate.
If you need assistance configuring MFA, or if you need to change authentication methods, please reference this KB article.