If your method does not work, it could be that one of the selected MP3 files (to which you are trying to join the audiobook) is not a spoken word but contains elements of music.
I recommend you use the conversion Method or find out what MP3 file is the problem.
In this method, you can create audiobooks from MP3 files.
As an mp3 to audiobook converter, iTunes is great. Although the created audiobook files are removed from the iTunes library, you can import them from the local to iTunes or other players with audiobook functions. And it is not necessary to reuse the original MP3 file as long as you have stored the created M4B audiobook files.
Follow the instructions here …