Why can’t I spend money that I just put on $1500

Wormie,

First did you verify that the redemption was successful, and you now have that amount in you credit balance?

If so, you should be able to use it for in-app purchases. See information here: What you can buy with your Apple ID balance - Apple Support

if there is still a problem, contact Apple Support.

If the card successfully redeemed, you can go ahead and use the credit balance to buy content. The credit balance cannot be used to buy gifts of any kind.