Your problem is that you are using the body_entered function, meaning it only gets called once, when the collider enters for the first time. I take it that you basically want that function to keep getting called while the player stays in the area.
Now, I don't know GDscript but I searched it up and a good fix seems to be to use a true/false that keeps track of whether or not the player is inside the area. Then repeatedly checking if it is true:
Make a new boolean called "touching_player" or something similar. Then use the body_entered and body_exit to set it true or false. Then, in the main loop, if touching_player is true then do the enter key stuff.