I've tried every conceivable permutation of things unfortunately.That’s really strange.
Have you tried signing out of your account, adding the game to your cart and then signing into your account to see if that does anything? Maybe the system will allow you to purchase the game by doing that.
Thats been an issue with sony since the dawn of time. I believe if you contact support they'll remove the license from the PS plus version and allow you to purchase. Its stupid af.
Truly baffling they would obfuscate something where I'm trying to give them money.
Amazes me this could be an issue this long into the service. Miles behind Microsoft in this regard.