Thanks for clarifying giolinke,
Have you made sure when downloading music that you are on Wi-Fi and Low power Mode is turned off?
We suggest following these steps to see if they resolve the issue.
Add music to Apple Watch - Apple Support
"Add music using your iPhone
- Open the Apple Watch app on your iPhone.
- Tap My Watch, then tap Music.
- Below Playlists & Albums, tap Add Music.
- Select albums and playlists to sync to your Apple Watch.
In watchOS 9.1, music is added when Apple Watch is placed near your iPhone. In earlier versions of watchOS, your Apple Watch must also be connected to power.
Tip: Use the Music app on your iPhone to create playlists specifically for music you want to listen to on your Apple Watch—music that motivates you during a workout, for example. See the Apple Support article How to make a playlist in the Apple Music app.
Add music using your Apple Watch
If you’re an Apple Music subscriber, you can add music using your Apple Watch.
Note: Adding music isn’t available in the Apple Music Voice Plan. For more information, see the Apple Support article About Apple Music Voice.
- Open the Music app
on your Apple Watch. - Tap Listen Now or Search, then navigate to music you want to add.
- Tap a playlist or album, tap
, then tap Add to Library. - A message confirms that the item was added.
- Note: You can stream music you add to Apple Watch when you have an internet connection. To play music when you’re not connected to the internet, you must first download it.
- To download the music to Apple Watch, tap
again, then tap Download.
Add a workout playlist to Apple Watch
You can add a playlist from your music library that plays automatically when you start a workout in the Workout app on Apple Watch.
- Open the Apple Watch app on your iPhone.
- Tap My Watch, then tap Workout.
- Tap Workout Playlist, then choose a playlist.
The playlist is added to My Watch > Music in the Apple Watch app on your iPhone.
Note: A workout playlist won’t play if you’re currently listening to other music or audio."
We hope this helps!