Please how can I redeem an iTunes card sent to me by my friend in cuba

If you don't live in Cuba, then it's not possible.

See ->  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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Code must be redeemed in a different country or region

• If you see a message that says the code must be redeemed in another storefront, the card was probably purchased in another country or region. You can't redeem Apple Gift Cards or App Store & iTunes Gift Cards outside of 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.

• If you need more help, contact Apple Support.