Apple Music relies on 2 components of your Music folder to properly sync music and playlists

Music Library file - Contains Playlist Names, Apple Music Settings, and Music Information

Media Folder - Contains all your music files

If you are missing music files or playlists in your Apple Music Library. Follow these steps.

1. Make sure your Music Library file is located in your Music sub folder under the Main Music folder on your Macbook

2. Make sure all your music is located in either Automatically Add to Music or the Music folder. Both folders are located under Media, in the same place as the Music Library file.

If you are still missing files. Enable these 3 settings in Apple Music

Enable Keep Music Media folder organized and Copy files to Music Media folder when adding to the library

This will add any new music that you have downloaded or imported from a CD into the Media folder.

NOTE: If you download/import music on your Macbook. It must be played at least once to show up in your library in Apple Music for the Copy setting to work.

And

Enable Consolidate files

Consolidating will make copies of all your music that is scattered across different areas on your computer and centralize them into your Media folder. 

--If you are still missing music files at this point, then the music file is no longer on your Macbook---

Known Issues:

If the Music Library file is located outside of the music folder, Apple Music will create a new one after a Macbook reboot. Resulting in playlist and library info to be empty. Which is why step 1 is important.