I don't have any other thing named CustomizeGui but here is my script and other stuff. There are some screenshots. The links are screenshots to give some context because the title isn't the best worded title I had in mind.
First link: https://ibb.co/j4yRzzN
Second link: https://ibb.co/Bw3qnQY
local textButton = script.Parent
textButton.MouseButton1Click:Connect(function()
local frame = script.Parent.Parent.Parent.CustomizeGui.Frame
frame:TweenPosition(UDim2.new(0.879, 0,0, 0))
frame.Visible = true
end)

6 comments
The first screenshot I changed the ScreenGui to ClassPickerGui
Also ClassPickerGui is a Gui
local frame = script.Parent.Parent.Parent.Frame
the 3rd parent is already the CustomizeGui, so there is no need to search for itself