The voucher code you entered is not valid.
Most common reasons are:
1) Code has been entered incorrectly.
2) The code you are trying to redeem has expired or is not active yet.
3) Your PSN account region does not match the voucher's region.
4) Your account does not meet the condition that the voucher is limited to.
1) Make sure that the code is entered correctly.
2) Check if the code's date is valid, and also check if the voucher has any restrictions of usage.