Make sure both devices are using Apple devices
Make sure both devices have iMessage turned on
Connect to Wi-Fi or mobile data
Check that the recipient’s phone number or Apple ID is registered with iMessage
Open the Messages app
Start a new message or open the existing conversation
Type the recipient’s Apple ID email or iPhone number
Send the message
Confirm the message bubble turns blue
If it stays green, restart the device and try again
If needed, sign out of iMessage and sign back in in Settings
If needed, update iOS to the latest version