attempt to call field 'PromptGamePassPurchaseFinished' (a userdata value)? [URGENT! PLEASE HELP!]

PromptGamePassPurchaseFinished() Is an Event, not a function. If you want to prompt a Gamepass you would use PromptGamePassPurchase(), which is a Function. You can read about it here: https://www.robloxdev.com/api-reference/function/MarketplaceService/PromptGamePassPurchase

Here's a Code example:

marketPlaceService = game:GetService("MarketplaceService")

script.Parent.MainShop.Invincibility.BuyPrompt.MouseButton1Click:Connect(function()		
	marketPlaceService:PromptGamePassPurchase(player,5094834) -- Prompting the Gamepass Purchase
end)

marketPlaceService.PromptGamePassPurchaseFinished:Connect(function(Player,Id,Result) -- Getting the result of the purchase
	if Result == true then
		print("Bought!")
	else
		print("Not bought")
	end
end)