Console Redemption Error: (WC-40378-2) - "Rate Limit Exceeded"
Web Redemption Error: "The prepaid card could not be redeemed. Please try again later"
This means that Sony have locked your ability to redeem codes for a 24 hour period which starts from the last time you attempted to redeem a code. For that lock to lift you'll need to wait 24 hours without redeeming a code.
Unfortunately, Green Man Gaming are unable to assist with Sony account lockouts. We can only suggest that you wait before attempting to redeem your key again or contact Sony Support for further information.