If you are like me and you have been working along the tutorials by saving over the start file up until this point, you'll get this error because you have not implemented the code you created in the steering behaviors project. The file "steering.gd" in the Autoload folder of that project can be copied without modification into the Autoload folder of your platformer project. Simply add it to the autoload settings in your project like you did in the Steering Behaviors project and the error will cease.
Did I forget to mention that in the videos? If so, I'll probably add the Steering.gd file to the start project.
Yeah, it's not clear from the previous chapter that this file had to be included. :P