When password strength is set to Strong under System > Settings > Security > Setup, the Customer Control Panel still allows creating users with weak passwords.
This is software issue #PPA-1149. The fix is planned to be included in one of the future product updates.
Update the password strength for Plesk backend users manually:
# /usr/local/psa/bin/server_pref --update -min_password_strength strong
Note: possible values for
very_weak weak medium strong very_strong