Check these first
- Copy the code from the order page rather than typing it. Zero and O, one and I, are the usual culprits.
- Check for a space that came along at the start or end when you copied.
- Region. A code for one country will not redeem on an account registered in another.
- The right redemption page. Each service has its own; redeeming on the wrong one gives a generic "invalid" error.
- The currency of your account. Some wallets only accept codes in their own currency.
What to do
If it still fails, use Report a problem on the order, choose not working, and attach a screenshot of the exact error message from the redemption page. That screenshot is what settles the claim.