How can I enable / activate my Apple Gift Card?

See if this helps.

How to redeem your Apple Gift Card or App Store & iTunes gift card - Apple Support

If you have an Apple Store Gift Card, it may only be able to be used for store or online purchases. This may help to identify the gift card you have.

What type of gift card do I have? - Apple Support

The link provided step by step instructions, what did not work for you?