Official Quote from Sony Press on the Sony PlayStation Blog:
We are aware that users are experiencing some disruption to the PSN service. To minimise the inconvenience we have suspended the “redeem voucher” functionality whilst 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 stage. Other PSN features such as log-in, online multi-player gaming, PlayStation Plus trial, PS Store (excluding voucher redemption), trophies, messages, friends etc. are all available.
We apologies and will provide an update to this announcement soon.
We just can KEEP CALM 😉