Open the Messages app on your Mac
Sign in with your Apple ID
Enter your Apple ID and password if prompted
Verify your account with the two-factor authentication code if required
Go to Messages in the menu bar
Select Settings or Preferences
Click the iMessage tab
Make sure Enable this account is checked
Confirm your phone number and email addresses are selected
Set your preferred email or phone number for starting new conversations
Open the Messages app on your iPhone if needed
Go to Settings
Tap your Apple ID
Tap iCloud or Messages and ensure Messages in iCloud is enabled if desired
Make sure your Mac is connected to the internet
Restart the Messages app if iMessage does not activate immediately