I’m having the exact same issue here, i tried restarting, restoring licenses and made sure my PS4 is activated as primary. Still just says “not available for purchase”. Super frustrating.
Edit: found a thread on r/diablo4 with PS5 users experiencing the same thing… doesnt sound like the issue was ever fixed by Sony either. I guess they dont want us buying the game huh
I’ve found a workaround for anyone who stumbles on this thread: login to your account on the playstation store website or app. I was able to purchase the standard edition there and then download the game from my library.