iPhones can only send SMS text messages, to non-Apple devices. This is a service, as you may know, that is provided by your cellular voice provider.
There a two potential reasons that you cannot send a SMS texts:
- The recipient has blocked you.
- There is something amiss with your SMS service.
Also, the support article you arrived here from should provide you with some troubleshooting steps. Have you tried them all yet? If so, and they didn't help, I suggest that you first contact your cellular provider, and if needed, contact Apple Support for further assistance with this matter.