When you use preload("res://TSCN/Item.tscn"), you get a PackedScene. It is like a schematic to create the scene. Therefore, there is no single instance, but as many as you want.
When you call instance() on your PackedScene, it returns a Node, with potentially other ones attached to it. To display them, you have to put them into the main tree using add_child() on a Node present in the tree. Note that adding the top Node will by extension add it's children "automatically".
If you run code inside your onButtonpressed, it will only be run once per press. To make tex follow your cursor, you can run it inside a _process().
In short, you get :
onready var tex = preload("res://TSCN/Item.tscn").instance() //Not the PackedScene, but an instance of it
var follow := false
. add_child(tex) //Not the PackedScene, but an instance of it
. follow = true
. if follow:
. . tex.position = getglobalmouseposition()