I bought it from the e-commerce website cooping here.

You need to follow the steps in the support document you posted from to update Proof of Purchase. We cannot do that for you here, in this user to user forum.

Link-> https://support.apple.com/ko-kr/HT202703?

In order to help we need information. We are just other users here.

We do not know what you bought and if you bought it used or from some third party, the warranty period may have expired.