What’s the grammatical error in had know?

What kind of error is it?

This is a grammatical error involving the use of incorrect verb tense. Specifically, it is an error in forming the past perfect tense.

Why do people make this mistake?

People often make this error due to confusion between "know" as a present or past simple form and "known" as the past participle. The structure of past perfect tense requires the past participle form, not the bare infinitive or present form of the verb.

What is correct?

The correct form is "had known," where "had" is the auxiliary verb and "known" is the past participle.