That is happening because you are in the wrong place. You can't fill out your party because the game went to the Elven Capital, but your party is in the "Leon Woods". Look for the Leon Woods tab in the header and click on it. Or you may need to go to the travel menu in the mansion. Either way, you ned to get the game focus back to the Leon Woods. This happens in other quests as well. It is a bug, but one that can worked around.
Viewing post in Quest bug when ambushed by Leon
I called that save a dead save awhile ago.
Cause I was screen locked on that page.
I was automatically dragged to that screen from the previous dialogue and I couldnt get out of it.
Based on what I thought the problem was, I got there again and it all worked fine, so not sure about anything else other than a random bug or the issue I thought it was because I didnt change anything else