Error Message:
"You must choose a strong password." occurs while creating Microsoft 365 users.
As the admin of an organization, you're responsible for setting the password policy for users in your organization. Setting the password policy can be complicated and confusing, and this article provides recommendations to make your organization more secure against password attacks.
Possible Cause:
The above error shows up if a password is entered which does not meet the complexity requirement in Microsoft 365.
Resolution:
1. You can set the up the manual password and make sure that it meets the password complexity requirement by including a (A-Z, a-z, 0-9, %^&*!@#) characters.
2. If you are using random password option, then you can navigate to Admin > Password Policy > Set the Password which meets the requirements.
You may also refer to the below link for reference,