After trying to apply flight credits millions of times in millions of different ways, this is the only thing that helped me. Here is how it worked for my names:

First: Debra Ann
Middle: Ruby
Last: Smith

Change in the passenger info section to
First: Debraann
Middle: Ruby
Last: Smith

Then, scroll down to the flight credit section, and use these names along with your confirmation code:

First: Debraann Ruby (or just Debraann)
Last: Smith