Hi

If any of the tracks in your playlist are located in the cloud (such as from Apple Music), check that you have downloaded all of the songs to the selected playlist on your iPhone (for offline listening) before syncing the playlist with your watch - and also that you are following all of the other steps here:

Listen to music on your Apple Watch - Apple Support

Add music to Apple Watch

Store songs on Apple Watch. Open the Apple Watch app on iPhone, tap My Watch, go to Music > Synced Music, then choose a playlist or an autofill option.

When the sync finishes, you can open the Settings app on Apple Watch, go to General > About, and look under Songs to confirm the number of songs copied to Apple Watch.

Tip: Use the Music app on iPhone to create a playlist specifically for music you want to listen to on Apple Watch.

Limit the songs stored on Apple Watch. Open the Apple Watch app on iPhone, tap My Watch, go to Music > Storage Limit, then choose a storage limit or maximum number of songs to be stored on Apple Watch.

See how much music is stored on Apple Watch. On Apple Watch, open the Settings app , go to General > About, and look under Songs.

Information above is from the Apple Watch User Guide >  https://help.apple.com/watch/#/apd483798d11

How to play music here >  https://help.apple.com/watch/#/apd70768b20b