It’s possible that your full name is “Mark” (with no last name), there’s no way for Discourse to verify whether or not that’s true.
If your forum has a policy that requires users to use their full legal name (or at least a part of it) that can’t be enforced by software. Perhaps instead of adding “First Name” and “Last Name” you add a checkbox to confirm that the user has “entered my legal name into the full name field”.