There are a few methods you can use to send the messages such as iMessage, send as SMS, and there is also MMS messaging. It is vital to have all these set to on. In order to check, go to Settings, and tap on Messages.
If you have too many saved messages you exchanged with someone, you should delete them. If you want to keep something, take a screenshot. Once you delete the conversation, try sending the message again.
his can help: go to Settings > Messages and turn iMessage on/off. After a couple of seconds, just turn it on again, and see if it will make a difference.
Click on Settings > Messages > Send & Receive. You will see the Apple ID, click on it, in order to sign out, and then, sign back in.
To update the carrier settings, go to Settings, General and choose About.
To update iOS, first, connect your iPhone to a power source, connect to a Wi-Fi network, and then tap on Settings > General > Software update.