Agile web Development with Rails section 9.3

Hello I am reading Agile Web Development with rails (third addition). And I need some help with it. I have reached chapter 9.3 (AJAX)

when I press on the add to cart button I get an error message. a real ugly one.

eventually, I downloaded the PDF version and copied the content of the files but it did not help.

I have looked but could not find the problem. Please give me a hand.

the files of this application are at: Download link:


I think that the error is written in javascript and this is it: