Why "buyed" isn't in your dictionary

What kind of error is it?

The word "buyed" is a grammatical error. It is an incorrect past tense form of the verb "buy".

Why do people make this mistake?

People might use "buyed" by applying a regular past tense formation rule (-ed) that works for many English verbs, but "buy" is an irregular verb and doesn't follow this rule.

What is correct?

The correct past tense form of "buy" is "bought".