I was having this exact same problem, I think this might actually be a PSN issue and not a SE issue. I got around it by, right after entering my code and clicking "confirm" spamming the "x" button cause I could see that the confirmation of the code working WAS popping up. Just the store is just not working right and is more or less crashing to the main menus. Spamming "x" still made me go back to the menus but FFXIV was in my Library when going back into the PSN! And putting in the PS3 disc doesn't work, it's unreadable by the PS4. This definitely seems like a PSN issue.