Why is my output saying to use a nil value???

I am new to scripting and I am teaching myself which is going smoothly except the output is saying I need a nil value to fix my script

local play = script.Parent
local Frame = game.StarterGui.Check.Frame
local Click = play.MouseButton1Click:connect(onClick)
local Check = game.StarterGui.Check

Check.Frame.Visible = true

function onClick(play)
Check.Frame.Visible = nil
end

the output says attempt to call a nil value

I even tried making the script:

local play = script.Parent
local Frame = game.StarterGui.Check.Frame
local Click = play.MouseButton1Click:connect(onClick)
local Check = game.StarterGui.Check

Check.Frame.Visible = true

function onClick(play)
Check.Frame.Visible = nil
if nil == true then
    Check.Frame.Visible = false
end
end

Please help!