The error 500121 means that the user account is locked out. This can happen if the user for some reason is unable to complete the MFA prompt, or if there have been too many failed sign-in attempts.
Some things to try:
- Re-registering MFA, revoking MFA sessions, deleting the authenticator app and re-installing it (if applicable)
- If Authenticator is installed, confirm if the OS & Authenticator app version installed are the latest one
- Confirm whether there are any Conditional Access policies being applied (and if so, test by excluding the user from the policy)
- Try clearing the browser cache or using a fresh browser.
If there are any additional logs you can share, that would be helpful as well.
If you are completely locked out and there is no other global admin in your tenant who can let you back in, please contact our hotline to get assistance getting back in: Global Customer Service phone numbers (microsoft.com).