If you see a red exclamation point
If you see or "not delivered" next to a message, tap the alert to try sending the message again. If the message still won't send, try the steps below.
If you can't send or receive messages
After each step, try to send or receive a message on your Apple Watch again.
Check Control Center
If you see the Do Not Disturb or Airplane Mode icons on your watch face, you need to turn the settings off in Control Center.
Swipe up on your watch face, then tap the highlighted icons to turn off Do Not Disturb or Airplane Mode.
Check your connection
Make sure that your Apple Watch can connect to your iPhone, cellular, or Wi-Fi. To send and receive SMS/MMS from your Apple Watch Series 3 (GPS + Cellular), your paired iPhone must be powered on and connected to Wi-Fi or cellular, but your iPhone doesn’t need to be nearby.
Even if your iPhone isn’t connected or powered on, you can send and receive iMessages on your Apple Watch when your Apple Watch is connected to Wi-Fi or cellular.
Try sending a message from your iPhone
If your iPhone can't send or receive messages, try to fix messages on your iPhone.
Make sure that you signed in
On your iPhone, open the Apple Watch app, tap the My Watch tab, then tap General > Apple ID.
Check your iMessage settings
On your iPhone, go to Settings > Messages and make sure that iMessage is turned on.
Then tap Send & Receive and make sure that you're using the same Apple ID that your Apple Watch is using.
If you aren't signed in, sign in to iMessage with your Apple ID.
Sign out of iMessage
Tap Settings > Messages > Send & Receive. Tap your Apple ID, then tap Sign Out.
Then tap Settings > Messages. Turn iMessage off, then turn it back on
Sign in to iMessage again
Tap Settings > Messages > Send & Receive > Use your Apple ID for iMessage, then sign in.