Script works in studio but not in a regular server?

The problem with this script is that the script is working on the server and attempting to make changes to the client (which includes UI).

This cannot happen without the help of a RemoteEvent.

Here is what your script should look like:

local admins = {

    21063444;
	-1; --For testing inside of Studio

}

local function IsAdmin(Player)
    for _,Admin in pairs(admins) do
        if type(Admin) == "number" and Admin == Player.UserId then
            return true
        end
    end
end

game.Players.PlayerAdded:Connect(function(Player)
    Player.Chatted:Connect(function(message, recipient)
        if not recipient and IsAdmin(Player) then
            if message == "!Admin" then 
                game.ReplicatedStorage.Toggle:FireClient(Player)
            end
        end
    end)

end)

Find ReplicatedStorage and place a RemoteEvent in there, and name it "Toggle".

Next, create a LocalScript inside of Main. This is what it should look like:

game.ReplicatedStorage.Toggle.OnClientEvent:Connect(function()
	if script.Parent.Visible == true then
		script.Parent.Visible = false
	elseif script.Parent.Visible == false then
		script.Parent.Visible = true
	end
end)

The reasoning behind this is that when then RemoteEvent is fired (line 20 of the first script), a LocalScript on the client can receive the signal and create a reaction (basically the whole LocalScript) since LocalScripts can make changes on the client and regular scripts cannot.