After migrating from Drupal 7 to Backdrop 1.14 I get the following error when I enter a post from any book:
Notice: Undefined index: book_mode en book_block_view() (line 269 /..../core/modules/book/book.module).
I was previously given another error related to books because I had not correctly migrated the configuration of the books module, a tight time gives me the previous error, does anyone know what it can be?
Thank you.
So you have the correct settings on /admin/content/book/settings now, but the Notice is still there... hm...
Did you already try to flush all caches? (You probably did, but just in case...)
So the problem might be with your default layout settings.
If you go to /admin/structure/layouts/manage/default and open Configure block "Book navigation" - what are the settings for "Book navigation block display"?
If you save that block settings and save the layout afterwards: does the Notice go away?