Turning off 2fa authentication for the entire tenant and the users.
Login to the tenant as an organizational admin, or tenant admin.
This is probably happening due to the security defaults for O365 which you can in fact turn off.
1. Go to portal.azure.com
( under the tenant management )
2. Use the search bar on the upper middle part of the page and search of "Azure Active Directory".
3. Under Azure Active Directory, search for Properties on the left-hand panel. It is in-between of User Settings and Security.
4. Under the Properties, click on Manage Security defaults.
5. Under the Enable Security defaults, toggle it to NO.
6. Wait for few minutes for propagation then try to sign-in using InPrivate or Incognito.
After trying this - it should no longer require MFA.