When adding MSSQL Database User, you receive the following error:

Unable to add database user. Please try again or contact support.
or
The username/password you've entered for this new database user is invalid. Please try again.

To avoid the error please use stronger passwords while creating a database user.

The Password must meet complexity requirements:

Be at least six characters in length
Contain characters from three of the following four categories:
English uppercase characters (A through Z)
English lowercase characters (a through z)
Base 10 digits (0 through 9)
Non-alphabetic characters (for example, !, $, #, %)
Complexity requirements are enforced when passwords are changed or created.