Change the product ID in the script that prompts the purchase?
Check if the product/gamepass is in the same place (universe) where you are trying to sell it? (For this you would need to enable “3rd party sales” in the game settings.)
Probably not the problem, but make sure you’re using MarketplaceService:PromptProductPurchase and not anything related to gamepasses or MarketplaceService:PromptPurchase