i don't know the script to make a purchase button?

You can prompt a purchase for players to purchase a gamepass or some other dev product, but you cannot force someone to purchase a the product. That is up to them.

Although, if you want to bring up the purchase prompt and don't know how, this is how you'd do that:

-- *****This code should be within a 'LocalScript' object****
local MarketplaceService = game:GetService("MarketplaceService")
local Players = game:GetService("Players")

local productID = 0000000  -- Change this to your developer product ID

-- Function to prompt purchase of the developer product
local function promptPurchase()
    local player = Players.LocalPlayer
    MarketplaceService:PromptProductPurchase(player, productID)
end


--function for when the player clicks a button goes here....
    promptPurchase()
--end of button click function