When calling MarketplaceService:PromptGamePassPurchase, is it necessary to enable third-party sales?

the game and gamepass are created by the same group.

I would assume, that it is a no.

But if you want to test in studio, you should enable studio access to API.

oh, I’m going to try in the game server, thanks a lot!

No. ROBLOX will know when you have created gamepasses for that game.