Your password must:
- be 10 or more characters long
- have at least 7 unique characters
- not be your username
- not be this site's domain name
- not be a single dictionary word
- not be in the list of cracked passwords held at Pwned Passwords, your password is rejected if it is in this list of passwords.
This xkcd cartoon illustrates the problem with the password rules which are used by the vast majority of websites.
What it tells us is that password length matters far more than using non-alphabetic characters in our passwords.