When you sign in to Microsoft 365 for the first time, you will need to authenticate yourself using another method (SMS to mobile or call to landline number) in addition to entering your OST password. Please have your mobile phone ready for the following instructions!
The easiest way to do this is to have an SMS sent to your mobile phone during setup:
Optional
You can also use multi-factor authentication with the Microsoft Authenticat mobile app instead of SMS authentication. Once set up, this method is faster and safer!
How do I set up multi-factor authentication with the Microsoft Authenticator mobile app?