If you want to enable MFA for community user, you can able to do it by session setting in profile.
Please follow the below steps
Go to setup - > session setting (quick find box) - > High Assurance.
Under High Assurance MFA should be selected. Then go to community profile, click Edit
go to Session setting ->"Session security level required at login"-> choose High Assurance. -> save
Now you have enabled the MFA for the Community profile user. Here is the video for the reference.
