Translated by google:
>>Apple Watch won't update its software. watchOS 26.6 fails to install with the error "Unable to Install Update." I have already tried: charging above 50%, Wi-Fi, restarting, resetting network settings, and unpairing/pairing as a new Apple Watch without restoring a backup—yet the same error persists every time.<<
Did you already try these suggestions?
...
Try these steps:
- Make sure that your Apple Watch is connecting properly to its charger.
- Restart your Apple Watch.
- Restart your paired iPhone.
- Try again to update your Apple Watch.
If the update won't start:
- Open the Apple Watch app on your iPhone.
- Tap General > Storage.
- Delete the update file.
- Try again to update your Apple Watch.
...
copied from: Update your Apple Watch - Apple Support
Also make sure not to use any VPN, so that the Apple servers can be contacted by your devices.