Why does an Apple iPhone 7 unable to send or receive text messages (SMS) and how to fix it?

 Hard reboot your iPhone

A hard reboot simply resets the phone by forcing it to shut down and then restart normally. This process is very helpful for minor firmware and app errors, therefore you should try it for this problem as well. Here's how to do it:

1. Hold down the On/Off button for a few seconds.
2. Press and hold the Home key while holding down the On/Off button.
3. Keep both keys pressed until the screen goes off and on again.

Restore your iPhone 7

1. This is the end of the line in terms of troubleshooting. Many of the issues can be resolved using this way because iOS is more reliable than its competitors, but there's no guarantee, which is why Apple experts are still waiting for people to walk in. So, to wrap up your troubleshooting and keep the possibility of resolving the issue alive, restore your iPhone 7.