There are several reasons why your MS Authenticator may not be working.

Some of those reasons include:

1. If you have not used the Backup within the MS Authenticator App and you change phones
2. If you have a new phone and did not set up the MS Authenticator properly
3. Your MIU M365 username changed

Please click on this link to get a list of instructions for fixing this issue: MS Authenticator not working on my phone