My Apple gift card says code not valid. Where do I got to redeem it

Return it to the vendor from which it was purchased. It likely was not validated at the point of sale.

M1 Mac mini, macOS 12.0.1; Watch, series 5 GPS/cellular, watchOS 8.1; iPad Pro 2nd Gen, iPadOS 15.1; iPad Air 2, iPadOS 15.1; iPhone 12 Pro, iOS 15.1; TV 4th Gen, tvOS 15.1; HomePods, audioOS 15.1 and Airport Express w/ Airplay 2

No one here can help with that.

Take it back to where you bought it.

I need a new code for my apple gift card