First of all, why not use a tutorial that's made for the version you're using?
Using code from pre-3.0 requires many changes. As a beginner, you're not going to know how to make those changes.
For example, the
position property is a Vector2. That means that
set_position() takes one argument, a vector, not two, which is what you've written:
That code could read
But post Godot-3.0 direct property access was added (and greatly simplifies code), so you'd write
flare.position = position
Again, I'd strongly advise avoiding outdated tutorials when you're trying to learn the engine. There will be many things you just wind up learning to do the wrong way.