Open Settings on your iPhone or iPad
Tap Messages
Tap Send & Receive
Tap your Apple ID
Tap Sign In
Enter your Apple ID and password
Complete any verification prompts
Make sure iMessage is turned on
Wait for activation to finish
If needed, go to Settings
Tap your name
Tap Sign Out
Sign back in with your Apple ID
Restart your device if iMessage does not activate
Check your internet connection if sign-in fails