The program you’ve encoded your MP3 with hasn’t encoded it with the version of the metadata that supports chapter markers. This is common if you’ve exported it as an MP3 directly from Garageband. Fortunately, it’s an easy fix:
Songsmode. By default it’ll be in
Convert ID3 Tags.
Show in Finder.
This is a part of the Chapters experience I'm hoping to improve in future releases, so make sure your version of Chapters is always up to date! Still need help? Email me.