I'm getting error "E 0:00:02:0671 step: <Tween#-9223371994995620709>: started with no Tweeners."

    I would place the create_tween after the "if Input..". There's no point in creating the tween if you don't use it.

    Did you intend to use is_action_just_pressed?

    The duration of the tween is 0. Did you intend to tween modulate:a from 1 to 0?
    https://docs.godotengine.org/en/4.1/classes/class_tween.html#class-tween-method-tween-property

      TheGuyWithName Godot expect a tween to be started on the same frame it was created. Otherwise the tween object just get deleted. If your code doesn't enter that if block in a frame, you're creating the tween in vain, and you'll get that message. As @DaveTheCoder already said, put the tween creation line inside the if block.