できる and できている

Hello,

My textbook told me the grammar structure "まだ...していません" so I said to a Japanese person "まだ日本語が出来ていません" but he corrected me and said it should be "まだ日本語が出来ません."

Does anybody know why you can't say できていません? I thought maybe できる is never できている perhaps because it's a potential and not an action, but my Japanese friend informed me that you can say something like "英語ができている人"

Anyway, I would be very interested to hear the explanation if someone knows!

Right. できる is a state verb, so ～ている form is usually not used. However, できている has another meaning "to be completed", e.g., 準備ができている, 夕食の用意ができている.
英語ができている人 sounds odd to me, at laest as a meaning "a person who can speak English".