[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

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