[SOLVED]It says FireServer can only be called from client?

You can only use :FireServer() with LocalScript and only use OnServerEvent with ServerScript, and you can get player/change player stats with .Touched Example:

script.Parent.Touched:Connect(function(hit) -- Get hit part
	if hit.Parent:FindFirstChild("Humanoid") then -- If find humanoid
		local player = game.Players:GetPlayerFromCharacter(hit.Parent) -- Get the player from character
		if player then -- Check player
			print("Correct!")
		else
			print("Error!")
		end
	end
end)

Here is fixed script:

-- Removed RemoteEvent, you dont need this
local part = workspace:WaitForChild("potty button") -- Added a WaitForChild

part.Touched:Connect(function(hit) -- Changed Player to hit
   if hit.Parent:FindFirstChild("Humanoid") then
		local player = game.Players:GetPlayerFromCharacter(hit.Parent)
		if player then
			local stats = player:WaitForChild("leaderstats")
			if stats then
				local fullness = stats:FindFirstChild("Fullness") 
				if fullness then
					fullness.Value = 0
				end
			end
		end
	end
end)

If you change per client the server does not receive the information.

Wiki pages:

WaitForChild()

GetPlayerFromCharacter

Touched

Learn more of Remote Events / Functions: Remote Events/Functions

Hope it helped :D

Solved your problems? put in title [SOLVED] or accept a answer.