You're getting the error because
is_action_just_pressed() returns a
false). And, since it's not possible to subtract one
bool from another, you get the error you mentioned. Maybe you're looking for something like:
rotation.x = Input.get_action_strength("turn_left") - Input.get_action_strength("turn_right")
... or ...
rotation.x = get_axis("turn_left", "turn_right")