Keybind GUI : "On Button Clicked" procedure just don't work

I want the player to have a GUI that they access by pressing a keybind, and use it for trading items with potion effects when a button is clicked.

I've developed a similar system using a block and it perfectly works. However, since this GUI isn't tied to a block, I can't use the normal block inventory procedures.

I saw in a forum that another user had the same problem but nobody answered him, so please help me. Is it possible to make a GUI like this?

(i tryed a lot of procedure modification but the problem is the same it not works with a gui opened with keybind)