I bought an Apple Gift Card but it’s has only 15 digits instead of 16 digits and I can’t redeem it

You certainly still have the proof of payment, return the card and ask for your money back.

See also:  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

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Contact Apple Support.

You might need to provide:

• An image of the full front and back of your card.
• A sales receipt showing when your card was purchased.
• The serial number from your card. 

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support

Please follow the steps in this Apple Support article. 

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

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

I bought an Apple Card but it’s an only 13 digit instead of 26 what next