That error usually happens when Steam can’t connect to its servers properly. First, check your internet connection and make sure Steam isn’t blocked by a firewall or antivirus. Try restarting Steam or your computer. You can also clear the download cache in Steam settings (Steam -Settings - Downloads - Clear Download Cache). If it still doesn’t work, try redeeming the gift on the Steam website instead of the client.

If none of this works, contacting Steam Support is the safest option they can check the gift and your account directly.