Hello Edward Employee,
Thanks for your question
Your user needs to perform multi-factor authentication. There could be multiple things requiring multi-factor, e.g. Conditional Access policies, per-user enforcement, requested by client, among others
You will need to try an alternative sign in if you have one configured or If you have a Break Glass Account or alternative admin you can request they help disable the external auth.
If these aren't available you may need to create a support request: https://azure.microsoft.com/en-us/support/create-ticket
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola