Is it possible your texts are converting from sms to mms due to long messages or adding rich content.
If so you'll need to speak with your network provider to ask for your account to be provisioned for mms.
Sending mms needs a data connection and most networks charge to send mms.
Personally I use WhatsApp or Facebook messenger.
I use the Samsung Messages App to send sms.
This icon >
If this isn't the case then double check your message centre number in your phone's settings and also your apn settings.
Also speak with your Networks Customer Services just to have your account checked.
Another option is Settings > Apps > Messages > Storage > Clear Cache > Try again to send a message.
As a troubleshooting step try your sim card in another phone to see what happens then with sms.
If I can be of any further help then please don't hesitate. Take care. 😎
