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 steps demonstrate the procedure using iTunes for macOS. They should be similar when using iTunes running on Windows.
- 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.
- Connect your iPhone to your Mac/PC. Once connected, iPhone button is shown alongside the Audiobooks dropdown. Your iPhone is also shown in the sidebar.
- 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.
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. Else iTunes will warn about unavailable space.
Answer to your queries:
The iTunes version is current (12.8).
iTunes is set for manual synching.
The iPhone has enough space for an audiobook or books.
The iPhone does not have a SIM card.
This shouldn't affect your ability to add audiobooks.