Converting M4A file to MP3 - Selected files could not be found iTunes IOS 12.9 on PC

I want to convert M4A files (in this case, audiobooks) to MP3. This is what happened.


Changed the iTunes import settings to MP3.
Selected the book.
Selected File > Create MP3 version.


But iTunes displayed None of the selected files could be converted because they could not be found.

I did a general web search and found and tried two purported solutions:
 - Exit and reopen iTunes (did not work)
 - Use Ctrl-A to select all of the tracks (did not work)

In the next post, I explain what did work.
Another solution: 


Select View | As Songs to display the library as a list of tracks (i.e. not organised by album). This is the key part.
Then hold the Control key and select the songs to be converted by clicking each one.
Lastly, select File | Convert | Create MP3 Version.


Before doing the above, make sure that the sound encoder is set to MP3 under Edit | Preferences | Import Preferences | Import Using ...