When an error occurs in a script, the error page will be displayed:
The error message will give you a description of the error, along with the name of the script and line number where the error occurred.
This message indicates that there is an unknown field being references at line 4 of the calculate_cost script.
After taking a look to see what this script trying to do at line 4, we can see that one of the fields is missing on the form. Sure enough, the form is missing the field price_per_item.
