Grab your iPhone and launch the Apple Watch app there. Tap on General, then swipe down to Usage. Tap on Usage and you'll get a list of all the apps on your Apple Watch, as well as the amount of storage space you've used and what's left over and available to you. Delete the least used items to free up space.

https://discussions.apple.com/thread/250167219

Unpair your Apple Watch via the Watch app on your iPhone.
Then Re-pair your Apple Watch,this deletes temporary and corrupted files and also re-indexes the file system freeing up a surprisingly large amount of storage space.

https://discussions.apple.com/thread/251605882

Koennte helfen - hoffe ich zumindest