The account name you pick on the "Create Your Account" step of the first-boot setup wizard has to follow standard Linux username rules - the wizard rejects anything else before it ever creates the account.
Invalid Username:
- Must start with a lowercase letter
- Only lowercase letters, numbers and hyphens after that (no spaces, capital letters, or symbols)
- Under 33 characters total
Example: John Doe won't work, john-doe will.
Reserved username:
Some names are blocked because they collide with existing system accounts/groups (things like root, admin, daemon, etc.). Pick something else - your Full Name (shown on the lock screen and login) can still be anything, it's only the account/login name that's restricted.