See the following Apple articles:
How to redeem your Apple Gift Card or App Store & iTunes gift card
How to redeem your Apple Gift Card or App Store & iTunes gift card – Apple Support (UK)
Add money to your Apple ID balance
Add money to your Apple Account balance - Apple Support (IN)
I received a Apple gift card by email,
Check 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
👉 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 cannot add an Apple Gift card to Wallet.
You can add a credit, debit, or prepaid card to the Wallet, see the following Apple articles:
Set up Apple Pay
Set up Apple Pay - Apple Support
Countries and regions that support Apple Pay
Countries and regions that support Apple Pay - Apple Support (EG)
Set up Apple Pay in Wallet on iPhone
Set up Apple Pay in Wallet on iPhone – Apple Support (UK)