Works on Local, but not on Server?

I've got a simple script that when it is clicked it will prompt a product purchase. Here is a sample code for a R$10 purchase.

productId = 33054891


script.Parent.ClickDetector.MouseClick:connect(function(plr)
	game:GetService("MarketplaceService"):PromptProductPurchase(plr, productId)
end)

script.Parent.ClickDetector.MouseHoverEnter:connect(function()
	script.Parent.SurfaceGui.Ammount.TextTransparency = 0.5
	script.Parent.SurfaceGui.Sign.TextTransparency = 0.5
	script.Parent.Enter.TextLabel.Visible = true
end)

script.Parent.ClickDetector.MouseHoverLeave:connect(function()
	script.Parent.SurfaceGui.Ammount.TextTransparency = 0
	script.Parent.SurfaceGui.Sign.TextTransparency = 0
	script.Parent.Enter.TextLabel.Visible = false
end)

Thank you in advance.

Your main problem here is either that plr is not defined, or your code is in a server script when it really should be local.

Move all your code into a LocalScript, and define the variable plr (probably as game.Players.LocalPlayer)

If you need the LocalScript to change a value on the server, use a RemoteEvent.