Looks like you are getting error due to MFA.
Error Code: 500121
This typically occurs when authentication failed as part of the multi-factor authentication (MFA) request, it may indicate a configuration or service error. To resolve this issue, can you try resetting (require re-register) MFA for your user.
With this user will be asked to re-register for MFA. Once the registration is done for user, you can again try to login and it should be successful.
Note: You will have to use global admin account to login to Azure portal and select the option "require re-register MFA".
Let me know if you have any further questions on this.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.