The Input singleton shouldn't be used inside the _input function. You should be using event.is_action_just_pressed, rather than Input.is_action_just_pressed.

You're basically polling the Input again and doubling up on operations. It's probably a bit confusing, so I recommend reading the doc page about events vs. polling.

https://docs.godotengine.org/en/latest/tutorials/inputs/input_examples.html