Maybe you opened the book on your phone? Try a new book, if that doesn't work try:
If you’re having trouble getting library books on a kobo (quite probably a new Kobo because changing things typically causes problems), try deleting all authorizations - (on your computer and on your ereader, but keep the ereader connected.) Then go to your public library website and get a NEW book - (something about trying to open a book that you already took out somewhere, causes a glitch - for example if you previously opened a book on a tablet or phone, then that’s it - there will very very likely be no opening it elsewhere, so make sure it is a newly borrowed one. If you are out of new borrows, use adobe to return a book, close and reopen adobe so that the return definitely happens). Download the new book to your saving spot, and click it to make it open in adobe digital editions: Adobe will then PROMPT YOU for the authorization. Use the drag down menu to find your library’s license type, for me it is Overdrive. Then enter your (Overdrive) email address and password. This should cause the book to open AND it also should cause your connected ereader to become authorized at the same time. Click the back button at the top and then click your ereader, and then it’s gear sign to look to see if it is authorized. Even if it is not authorized, try dragging the new book onto it. This should work immediately (as it did for me), or it should ask for an authorization. You can then click on the ereader (not on all items) and see if you can open the book as it sits in the ereader area. If it opens there, you should be good.