Why "has forbade" is a forbidden form

What kind of error is it?

It is a grammatical error involving incorrect verb tense usage.

Why do people make this mistake?

This error occurs because "forbade" is the past tense of "forbid", and people may mistakenly use it as a past participle, thinking they can form the present perfect by combining it with "has" or "have".

What is correct?

The correct form is "has forbidden" for the present perfect tense, as "forbidden" is the past participle of "forbid".