Log into 365 and click users and enter into Multi-Factor authentication.
Select “service settings”
Configure any relevant settings.
Next is an essential step, else people wont be able to sign in.
Open Org settings under Settings.
Scroll down to “Modern authentication” and enable it.
Enable MFA for the user and it will pop up with a prompt in Outlook telling the user that more information is required but you can also tell them to go to aka.ms/mfasetup