Playlists won't sync after turning on Apple Music

With the latest version of iOS, iPadOS, and macOS, Sync Library is turned on by default. If it is turned on you cannot sync MP3 from Computer /Mac to iPhone

On your iPhone, iPad, or iPod touch

1. Go to Settings > Music.
2. Turn OFF Sync Library. If you don't subscribe to Apple Music or iTunes Match, you won’t see an option to turn on / off Sync Library.

If Sync Library is updating or turned off, you'll see a message at the top of the Library tab in the Apple Music app.

• Mac Book Air Retina 13" macOS Catalina
• iPhone SE with iOS 13.5 :- Does not use iCloud Library
• iPhone 6 with iOS 12.4.7 :- uses iCloud Music Library

This is a two Step Process (To sync music from this computer, turn off Cloud Music Library)

1. Apple Music: Import music (MP3) to Apple Music Library on Mac
2. Finder: Sync between Apple Music on Mac and iPhone

Watch the GIF for complete process

To sync music from this computer, turn off Cloud Music Library.

Watch the same GIF as a QT movie file uploaded by me on YouTube:- Transfer MP3 Music from Mac (Catalina) to iPhone (iOS13.3)