I talked to GSC and they said there can be issues with trying to add a new payee while processing the expense directly. Here is the fix:
Before trying to enter the expense, add the payee separately. When you are in the finance webpage, there is a tab at the top called "Participants". Click on that tab, and then click on the link that says
"+ Add Participant"
From there, add the Payee as you normally would, and then click on "Add Participant" at the bottom of the form.
Once the participant has been added, then go back to your expense tab, enter your expense, and the new Payee will show up as a possible Payee.
