I want to make a brick that you can click to purchase a dev product. How can I do this ?

Try this, and please use this script parented to a brick!

local ClickDetector = Instance.new("ClickDetector", script.Parent)
ClickDetector.Name = "\0" -- a clickdetector with no name
ClickDetector.MaxActivationDistance = 32 -- set the maxactivationdistance to 32 studs
ClickDetector.MouseClick:Connect(function(plr) -- establish a RBXScriptConnection.
    game:GetService("MarketplaceService"):PromptProductPurchase(plr, devProductId) -- change devProductId to ur devproductid.
end)

If this helps, please mark this as a solution.

Metatable out!