One problem is that
"_feed_prompt(food)" is supposed to match the name of a function, but since you are trying to define the argument, there is no matching function found. is
_feed_prompt a method on
Food? if so, you should just call it directly since you've already captured the "pressed" signal at this point.
var food = preload("res://Food.tscn").instance()
Although, I'm having a hard time understanding what this is trying to accomplish, so it's possible that it could be done some other way.
If you could share your scene/node structure, that would be helpful