Try these steps first
After trying each step below, try to send a message again.
- Restart your Mac. Choose Apple menu > Restart, then click Restart.
- Make sure your software is up to date. Find out how to update the software on your Mac.
- Make sure your Mac is connected to the internet. To check your internet connection, try loading a page in Safari or another web browser.
- Check that the date and time has been set correctly on your Mac.
- Make sure you enter the correct phone number or email address for the contact.
If you see a "This user is not registered with iMessage" alert
If your contacts see your email instead of your phone number
If you can send messages but your contacts see your email address instead of your phone number, make sure you're signed in to iMessage with the same Apple ID on all your devices. Find out how to add your phone number to Messages on your Mac.