I had a default notebook set on desktop but no notebooks showed in mobile when I tried to set a default notebook.
I found this worked:
Open Evernote desktop app.
Change default notebook to another (I happened to have a second).
Change default notebook back to original notebook
Open mobile app.
Select default notebook in settings.
If you don't have a second notebook, you might want to create (and later delete) one just to help resolve this issue.
