Please make sure check these out before you redeem your voucher:
A. You keyed in the code wrongly
B. You logged in to your steam account out of our permitted region
C. You are using a proxy which carries an IP address out of our region
D. some universities/offices might block certain port (including port uses by steam)
As for cases mentioned above, you will receive the message "The wallet code you entered is invalid" and please try again key in the voucher within 30 minutes after the first attempt. If it still occurs, please contact Steam Wallet Support.