Hello Phil.geerts,
For specific help with listening offline to your music from Spotify on your Apple Watch, we'd recommend looking over the steps from Spotify.
Spotify on Apple Watch
You will want to make sure your watchOS is version 7.0 or above. Also note they mention having paired Bluetooth headphones to your Apple Watch.
Listen offline
You can download at least 10 hours (depending on your model) of music and podcasts to Apple Watch so you can listen without the internet.
Note: There’s a limit of 100 downloads per playlist.
Download tracks to your Apple Watch from the Watch app
1. Swipe right to access the home screen.
2. Find a playlist you’d like to download, and tap Download.
If it still isn't working, and you do have paired Bluetooth headphones to your Apple Watch, you may need to remove the Spotify app on your Apple Watch, then redownload it. Start with these steps: Download apps on your Apple Watch - Apple Support
Reply back with any questions.
Take care.