StarterGui won't enable after touching a part?

Hello, your problem is that you're not detecting when a certain player is stepping over a part and you're accessing the StarterGui instead of the player's PlayerGui.

Here's the correct code. Put this in a script inside of your part.

local Players = game:GetService("Players")
local debounce = false --Prevent a ton of hit detections at once.

local function onTouched(hit)
	if hit.Parent:FindFirstChild("Humanoid") then --Checks if a humanoid hit.
		if not debounce then
			debounce = true
			local chr = hit.Parent
			local plr = Players:GetPlayerFromCharacter(chr)
			if plr then --Checks if it was a player. 
				if plr.PlayerGui:FindFirstChild("Buy1") then --Makes sure the player has the right gui.
					if plr.PlayerGui.Buy1.Enabled == false then
						plr.PlayerGui.Buy1.Enabled = true
						wait(3)
						plr.PlayerGui.Buy1.Enabled = false
					end
				end
			end
		end
		debounce = false
	end
end

script.Parent.Touched:connect(onTouched)

Hope this is what you wanted if it isn't tell me.