"Charging with this accessory is not supported by Apple Watch."

Hi

Try the following steps. When charging your watch, check that:

All plastic wrap has been removed from both ends and both sides of the charging cable (check closely on the head of the charger).

• The back of your watch and the head of the charging cable are clean and dry (clean them with a nonabrasive, lint-free cloth).
• The charging head is properly aligned with the back of the watch, with the concave / curved side touching it fully:

  Some watch docks, stands, cases or protectors might hinder this. If you are using any such accessories, try instead charging the watch by connecting it directly to the charging cable, without using them.

• If possible, test by using another genuine Apple Magnetic Charging Cable (for example, by borrowing one from a friend).
• If you are charging your watch by plugging the cable into mains power:
  • Check that the USB connector on the Apple Magnetic Charging Cable is pushed fully into the USB Power Adapter.
  • Check that the USB Power Adapter is pushed fully into the power outlet.
  • Try using different adapters from the following:
    • The Apple USB Power Adapter that was supplied with the watch, or;
    • An Apple 5W (iPhone) or 12W (iPad) USB Power Adapter, or;
    • An Apple-certified (MFi) third-party 5W USB power adapter.
  • Try using a different power outlet (one that is known to be working properly).
• If no other suitable power adapters are available, try instead charging by plugging the cable into a USB port.

  If charging via a USB port on a computer, ensure that you use a USB 2.0 or 3.0 port, the computer remains turned on and awake and the cable is plugged directly into the computer (not into, for example, an external keyboard).

It may also help to try these steps:

Restart your watch: Restart your Apple Watch - Apple Support

Unpair and re-pair your watch:

• The Watch app on your iPhone takes a backup of your watch automatically when unpairing via the app. Choose to Restore from Backup during set up. Most data and settings will be restored, with some exceptions (eg Apple Pay cards, passcode).
• Unpair your Apple Watch and iPhone - Apple Support
• Set up your Apple Watch - Apple Support

If you identify an issue with a particular component (adapter, cable or watch) or if the issue otherwise persists, contact Apple Support (mail-in service may be available) or make a Genius Bar reservation with a view to having your product(s) checked and, if necessary, serviced under warranty:

• Contact - Official Apple Support
• https://twitter.com/applesupport
• http://www.apple.com/retail/geniusbar/

More information:

Check your battery and charge your Apple Watch - Apple Support

Follow the instructions in this article If your iPhone, iPad, or iPod touch won't charge - Apple Support