First, see the 'If you have an issue when you download apps' section in:
Download apps on your Apple Watch - Apple Support
Second, make sure you have enough memory on the watch. To Free-Up Storage
For songs, see the "Remove music from Apple Watch" section in:
Add music to Apple Watch - Apple Support
and for pictures, see the "Limit photo storage on Apple Watch section in:
Choose a photo album and manage storage on Apple Watch - Apple Support
To delete apps on your watch, see:
Delete apps from your Apple Watch - Apple Support
Also, try unpairing and pairing your watch again and restoring from the backup just created in the unpairing process. This deletes all temp and corrupted files and re-indexes the file system. This should result in no loss of data and in some cases can free a lot of storage.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
Also see:
If you don’t have enough space to update your Apple Watch - Apple Support
If neither of those help, the first (and simplest) thing to try if something isn’t working right is to restart your Apple Watch and its paired iPhone. If necessary (device not responding), try a forced restart. Both methods can be found here:
How to restart your Apple Watch - Apple Support
Restart your iPhone - Apple Support
If that doesn't help, the next steps would be to unpair and pair your watch again and restore from the backup just created. This should result in no loss of data. It deletes all temp and corrupted files and re-indexes the file system.
Unpair and erase your Apple Watch - Apple Support
Set up your Apple Watch - Apple Support
If you still need help, contact Apple Support.
