Use an old iPhone 6 just for audiobooks - Cannot transfer file from iTunes to iPhone
Audiobooks doesn't automatically sync to iPhone upon connecting to iTunes. Neither can they be added to iPhone by simple drag and drop. Follow the steps mentioned below to get your audiobook on your iPhone. The key here is getting your audiobooks playing in iTunes before you attempt to sync them. If they don’t play in iTunes, this won’t work well.

The steps demonstrate the procedure using iTunes for macOS and should be similar when using iTunes running on Windows. If someone can upload windows screen shots, that might make a nice addition to the end of this procedure.


Make sure you have added the audiobooks into iTunes library or have downloaded the purchased audiobooks from the iTunes Store. You can check the same by selecting the Audiobooks dropdown in iTunes window. Play the tracks to be sure they work on the computer.





Connect your iPhone to your Mac/PC via USB or WiFi. Once connected, iPhone button shows itself alongside the Audiobooks dropdown. Your iPhone is also shown in the sidebar. If not, Apple support has articles on how to diagnose and fix when an iPhone won’t show up in iTunes.





Click on the iPhone button besides Audiobooks dropdown, select Audiobooks under Settings in sidebar.





In the main area, check Sync Audiobooks, select Selected audiobooks radio button and select the desired audiobook(s) and parts to sync.
Once the desired selection is made, click on Apply button shown towards bottom right. The sync progress will then begin - based on the size, it can take minutes or an hour or more. You can interrupt the sync - when iTunes is running and the phone connected, the sync will resume until it’s done.





Voila! Your audiobooks are now synced to your iPhone. Don't forget to eject your iPhone by pressing the eject button shown besides your phone's icon in the sidebar towards the top.
You'll need to make sure sufficient space in available in your iPhone. Settings on the phone and iTunes will warn about space running out should that happen.




Answer to your queries:


  The iTunes version is current (12.8).


Looks fine.


  iTunes is set for manual synching.


Looks fine.


  The iPhone has enough space for an audiobook or books.


Looks fine.


  The iPhone does not have a SIM card.


This shouldn't affect your ability to add audiobooks.