Why we can't say "I will can"

What kind of error is it?

A grammatical error involving the incorrect combination of modal auxiliary verbs.

Why do people make this mistake?

People make this error when trying to express future ability. They mistakenly combine "will" and "can", not realizing that in English, you cannot use two modal verbs together. Instead, the correct form to express future ability is "will be able to".

What is correct?

The correct expression is "I will be able to" instead of "I will can".