If you cannot sign in to the OverDrive website or setup the Libby app for Knox County Public Library with your library card number and PIN, first check that you've selected the correct library (there are other Knoxvilles and other Knox Counties in the U.S.A. so you can try searching by zip code for the library).
See if you can sign in at the catalog, and if you cannot, use the "Reset My PIN/Password" link there.
If that doesn't work, call Circulation at (865) 215-8751 and ask for an update of your account.