This is when a user can sign in with their credentials but is being blocked from authenticating MFA or error code 500121. the user will be prompted with MFA but then it is denied instantly this is where to unblock the users account to allow them to use MFA again
Log in to office 365 admin center go all admin centers> Microsoft Entra
Once in Entra scroll down to Protection> Multifactor authentication> Block/unblock users this is where you can unblock account that is being blocked by MFA sign in