How can I listen to my mp3 audiobooks on iPhone?
According to the support article for iTunes Match, tracks will not be uploaded to your iCloud Music Library if they:


are longer than 2 hours
are larger than 200 MB
"don't meet certain quality criteria"


The "quality criteria" include the bit rate, and I believe the same criteria apply with Apple Music.

You could try converting one of the files  to 128kb/s or higher AAC format, using the start/end time settings in the track's Get Info before converting if it's longer than two hours and/or if needed to get the resulting track under 200 MB. If that track still won't upload then something else is going wrong.

You can always sync files like this manually. On older macos and windows, add these to your iTunes library and sync via iTunes.


Use an old iPhone 6 just for audiobooks - Cannot transfer file from iTunes to iPhone


One newer macOS - add the files to music or podcasts app and sync over USB.