Redmi k40 sent a message indicating that it was not delivered.

1. Due to network reasons, users can check whether their mobile phones are in flight mode, or turn them off and restart, and then send short messages in places with good network signals;

2. the problem of mobile phone sim card. Take out the sim card of the mobile phone, see if there is anything wrong with the sim chip, and try again.

3, the phone has no phone bill, and you can't send text messages when you stop.