Why "I'm bias" doesn't say what you think

What kind of error is it?

It's a grammatical error involving the misuse of a noun instead of the correct adjective form.

Why do people make this mistake?

People often mistakenly use "bias" as an adjective instead of its correct form, "biased", due to similarities in pronunciation and a misunderstanding of word forms.

What is correct?

The correct usage should be "I'm biased," where "biased" is the adjective that describes the state of having a bias.