Sony’s eighth generation console, PlayStation 4 launched in Europe on November 29 and the arrival of large numbers of PlayStation users has made it necessary to disable Playstation Network.
PlayStation Network service is temporarily suspend to stabilize the network, Sony issues a statement regarding the outage:

“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.”

However, we have a work around to redeem Playstation Plus code, check the out below:

You’ll need PS3 or PS VITA with a main account, login to PS3 or PS Vita with the main account, redeem the code. All the benefits will be available on all your devices.
And If you want to redeem a non-PS Plus code, head over to Playstation Store WebPage, Sign-In, click on your PSN name and from drop down menu select “Redeem Code”.
These options above will help you redeem your voucher and link it to your main account on PS4.