Agile web Development with Rails section 9.3

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: