The problem with "I'm agree"

What kind of error is it?

Grammatical error - subject-verb agreement and verb usage.

Why do people make this mistake?

People often make this error due to direct translation from other languages where a similar expression uses the verb "to be" with an adjective or past participle. For example, in French "Je suis d'accord" or Spanish "Estoy de acuerdo," which literally mean "I am agree." Non-native English speakers may translate these structures word-for-word into English, leading to the error.

What is correct?

The correct expression is "I agree." In English, "agree" is an intransitive verb that does not require the verb "to be." It should directly follow the subject without any auxiliary verbs in the present simple tense.