Apple's iOS doesn't often run into serious problems. Most of the times the issues are small but at the same time irritating. One of such glitch is found that some users can't send a regular text message from the iPhone or iPad.
I am pretty sure you would have done the basic troubleshooting like checking the network connection. Even if basic troubleshooting has resulted in nothing, then you have landed on the right page. Just check the list of fixes given here and try them one by one.
How to Fix Can't Send Text Message from iPhone/iPad Issue
Reboot Your iPhone
Let's keep things simple in the beginning. Try restarting your iPhone and then give it a try by sending a text message to anyone. Most of the time software glitches are resolved by simply restarting. If this hasn’t solved the problem, then don’t worry, we have other fixes as well.
Delete Old Messages
Try deleting old messages from your iPhone. This might sound ridiculous, but garbage of old messages can be the culprit. So give it a try by going to Messages app → Edit → Select all the messages and then tap on Delete from the bottom right corner.
Toggle On Send as SMS
Before going through the hard way, make sure you have toggled on Messages under Settings. To check that, go to Settings → Messages → Toggle ON “Send as SMS”. A restart might be required, so better do that and check if you are able to send the text message.
Remove Country Code from Number
Few users over the forum have mentioned that they were able to send text messages after they removed the country code from the number they were trying to send a message. So it's better to give that a try as well, this might help.
Toggle Off Text Message Forwarding
If you have enabled Text Message Forwarding on your iPhone, then that could be creating a problem. Try turning it off by going to Settings → Messages → Text Message Forwarding → Toggle OFF Text Message Forwarding of all the listed device. After doing that, restart your iPhone and try sending a message.
Reset Network Settings
Sending a text message is related to network and any issue in network settings can be troubling the issue on the subject. Try resetting network settings on your iPhone by going to Settings → General → Reset → Reset Network Settings.
This will restart your iPhone, wait till it loads and then, try sending the message. If it doesn't work, keep reading below.
Use Data Only for LTE/Mobile Data
If your iPhone supports LTE or VoLTE, then they might be creating the problem. That's because not every area is capable of delivering LTE signal. It's better to turn it OFF or use Data Only. To do so, go to Settings → Cellular/Mobile Data → LTE → Data only OR turn it Off. It's not mandatory, but try rebooting your iPhone and then check if you are able to send a message.
Restore Your iDevice
The most irritating thing is to restore your iPhone. That is because you will lose the data you saved after making that particular backup. Anyways, when things don't work as intended, it's better to take the tough route. Give it a try by restoring your iPhone from the backup when you were able to send text messages perfectly. This should have definitely solved the issue.