Gravity Forms PayPal Error (and Solution): Some required information is missing or incomplete. Please correct your entries and try again.

One of our clients sells books on their website and was using a PayPal-generated Buy Now button to sell the book. However, they wanted to be able to sell multiple copies of the book so I added a form, using Gravity Forms with the PayPal Addon.

However, when I tested the form, it brought me to a page on Paypal with this error: