I bought an iTunes card and I can't charge it

1.

Otimize the range of the mobile network, see -> If you see No Service, Searching, or SOS on your iPhone or iPad

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

2.

See -> If you can't connect to the App Store, iTunes Store, or other Apple services

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

-> If you tried all of the above steps and you still can’t connect, contact Apple Support.

3.

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

Contact Apple Support -> Contact Apple Support.

See also -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support