Update: Digital voucher code redemption in PlayStation Store is returning to normal, thanks for your patience.
Update: We are continually increasing the availability of voucher code redemption functionality. We appreciate your patience and apologize for this inconvenience.
We are aware that users are experiencing some disruption to the PSN service. To minimize the inconvenience we have suspended the “redeem voucher” functionality while we investigate further. Unfortunately this means that money cards, product vouchers, PlayStation Plus vouchers, PS3-PS4 upgrade vouchers and any other vouchers for digital content are not redeemable at this time. Other PSN features such as log in, online multiplayer gaming, PlayStation Plus trials, PlayStation Store (excluding voucher redemption), Trophies, messages, friends, etc are all available.
We apologize for the inconvenience and will provide an update soon.