"does not allow"?
You are confusing the error report with WikiTree policy. The Error report is merely pointing out inconsistencies (and potential errors) in WikiTree.
The policy is that honorific titles should not be in the first name field, but the error report is unable to differentiate between a title and a person's actual name.
That's where we have to step up and "tell the error report", by marking it as a false error (as the others have already mentioned).