It's possible that the code entered was not scanned or entered correctly.

If you did scan the gift card/code, then there could have been an error during the characters recognized so double check all characters are correct. 
If you have typed the code yourself, verify there was no mistake made like confusing a 0 (zero) with the letter 'O', or other similar mistake (number 1 instead of letter 'I').

After you press the redeem button, iRedeemer will launch the iTunes Store app where the final redemption will be done. If you get any error message there then please let us know which one... Most likely you will have to contact Apple Customer Support about the issue since this is Apple's system handling the redemption of the gift card/code.