Hi, Now I'm in Hong Kong and I have an Apple gift card, but I can't activate it

I think you have answered your own question.

As you noted, Apple advises in If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card that You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside the country or region of purchase. For example, if you bought an App Store & iTunes Gift Card in France, you can't redeem it in the App Store in the United States.

You didn't mention where the card was bought but presumably it was brought into Hong Kong from another country and therefore is not valid in Hong Kong.

That explains why you can’t activate it. If you purchased the Apple gift card another country and are planning to cash the card then it must be activated in the same country that the card was purchased. This means that you will not be able activate the gift card in Hong Kong since you purchased the gift card in another region.

Axel F.