Did you try to redeem the code on your Playstation or through the website? Did you receive this message the first time you tried entering the code? Also, where did you purchase the Season Pass from?
I would suggest you try and check your download list.
You can check on it via logging in to https://store.sonyentertainmentnetwork.com/#!/
And then go to the drop down arrow and then go to download list and then check if the Season Pass is showing from there. If it is there, you may just need to download it.
If it isn't, I would suggest you contact PSN about the code and also go back to the Gamestop shop from where you got the game for assistance.