Replace PromptPurchase with PromptGamePassPurchase. PromptPurchase is a request to buy any single asset from the library (Game passes don’t appear in the library) and PromptGamePassPurchase is a request to purchase a game pass.
local productId = 12355653
local player = game.Players.LocalPlayer
script.Parent.MouseButton1Click:Connect(function()
game:GetService("MarketplaceService"):PromptGamePassPurchase(player, productId)
end)