i am a US citizen living in Japan. i do not have a japanese apple ID. i bought a apple gift card with yen. how do i apply it to my account

Trade, sell, or gift the card to somebody there who can use it with their Japanese Apple ID.

And please do not buy any more cards in yen. As Muguy notes, they cannot be used with your US account.

See this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card -Apple Support

check. ⤵️

Card is not valid

Card has already been redeemed

Card is not activated

Code is unreadable

Code must be redeemed in a different country or region

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.

Contact Apple Support -> Contact Apple Support.

You cannot. You can only add gift cards from the US