why aren't gift cards working immediately?

Gift Cards will only work after the retailer has activated them. When purchasing in store, they are activated at the register where they can be used immediately. It sounds like Amazon/Seller is delaying activation to make sure there is not an issue with your payment first. Contact Amazon for more details.

If the balance is in your Apple Account then it can be used immediately. I have not heard of there being any "waiting time". I have never bought a gift card from Amazon but they could conceivably have a wait time for their activating a card, but that would be before you could even redeem it. Because you already have account balance that implies you have redeemed your card and to do that Amazon would have had to have activated it.

We do not know what exact message your are seeing, and from whom. It could be that it says you may have to wait 48 hours, but activation typically takes much less time.

We also do not know where you are located. This could be something specific to your country only. Or maybe it is a third party vendor on Amazon selling these and they do things their own way?

From where and when was the gift card obtained and activated?

Have the activation receipt?

Are you attempting to redeem the gift card to your Apple Account?

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

Amazon delaying redemption to ensure payment “clearing” makes perfect sense …

… but the fact that it apparently HAS been redeemed (onto the Apple Account) might seem to discount that as a reason.

Unless the OP replies to the questions which have been posed, we’ll all be left w/o the “full story.”

So the balance is already reflected on your in-wallet Apple Account card.

Where are you trying to spend the balance and who (or what) is telling you “48 Hours” ?