Hello
Restart your Apple device:
- Turn off: Hold Sleep/Wake until red slider appears, then slide red slider right.
- Turn back on: Hold Sleep/Wake until Apple logo appears.
You can also try resetting the iMessages on the devices:
1. From the home screen, tap Settings.
2. Tap Messages.
3. Tap iMessage to toggle Off. Tap again to turn back on. Note: Please allow activation to complete before proceeding.
After iMessage reset, retry message.
Let us know if this fixes the problem!