The form was the refund form, not the cancellation form. DL doesn't like giving refunds so they aren't going to make it easy by pre-populating your information into the form.

I've seen the "not able to determine eCredit amount" message on some previous cancellations. Usually means that the original payment method was complicated (perhaps you booked it with a combination of eCredits and cash). You have no choice, you will have to redeem those over the phone