Hello @Joshua B,
Thank you for your time over call.
As we discussed, you were trying to register for MFA using the https://mysignins.microsoft.com/security-info page but encountered an issue during login because your personal account is linked with your GitHub profile.
To resolve this, we created another user account with global admin rights and tested whether you could register MFA methods with this new account. We successfully registered MFA for this user and made the necessary changes to your modern authentication method policies.
We also unchecked all verification options from the legacy per-user MFA blade and completed the migration process. Afterward, we enabled the Microsoft Authenticator method in the Modern Authentication Method Policy blade.
For your personal account, we used the link https://myaccount.microsoft.com/?tenant=<tenant_ID> to force authentication and direct you to the correct directory by specifying the tenant ID. This allowed us to successfully log in to the security info page and register MFA.
I hope this information is helpful. Please feel free to reach out if you have any further questions.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Thanks,
Raja Pothuraju.