Have you tried entering it on the website? That would be my first suggestion.
I know I had this issue back when Sony was having issues during the holidays last year, and it was a mess, but I never figured out why specifically.
The only explanations I can find from other posts on reddit and such are that it could be:
- Trying to add funds to a Sub Account instead of a Master Account
- Retailer didn't activate the card
- Server issues on Sony's end
So first try to enter it on the website, and if that doesn't work, make sure that it's not a sub account and it is a master account, and you still don't have it working, just call up PS support. They're normally pretty helpful.