How do I get a players ID from their username?

I am using a regular server script, which it can not get the users playerID.

script.Parent.MouseButton1Click:Connect(function(Player)
	local playerId = Player.UserId
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P1.Disabled = false
	wait(2)
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P1.Disabled = true
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P2.Disabled = false
	wait(2)
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P2.Disabled = true
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P3.Disabled = false
	wait(2)
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P3.Disabled = true
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P4.Disabled = false
	wait(2)
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P4.Disabled = true
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P5.Disabled = false
	wait(2)
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P5.Disabled = true
	script.Parent.Parent.No1.Value:WaitForChild('Eng').P6.Disabled = false
	script.Parent.Parent.Confirm.BackgroundColor3 = Color3.new(0, 255, 0)

In the developer console it was saying "attempt to index local 'Player' (a nil value)

You are using a gui. Guis do not have any way of getting the player as they are supposed to be manage by LocalScripts, which can access game.Players.LocalPlayer. Use a remote event.

LocalScript (Parented to gui button):

script.Parent.MouseButton1Click:Connect(function()
	local remoteEvent = game.ReplicatedStorage.RemoteEvent -- Make a remote event and reference it here
remoteEvent:FireServer(script.Parent)
end)

ServerScript (Parented to gui button):

game.ReplicatedStorage.RemoteEvent.OnServerEvent:Connect(function(player, gui) -- Reference same remote event
	local playerId = player.UserId
	-- Type the rest of the code here (replace every script.Parent with gui)
end)