Message reply failed when Apple Watch was used alone.

Check the Internet connection to ensure smooth network connection.

Check whether "do not disturb mode" or "Flight Mode" is turned on, press and hold the bottom of the dial, wait for the "Control Center" to appear, and then slide up. Click the highlighted "Do Not Disturb Mode" and "Flight Mode" icons to close the settings.

Make sure your AppleWatch can connect to iPhone, cellular network or Wi-Fi. To send and receive SMS/MMS on the AppleWatch cellular network model, the paired iPhone must be turned on and connected to the Wi-Fi or cellular network, but this iPhone does not have to be carried with you.