@clau12: If you're having trouble sending text messages, then please try enabling Mobile Data by swiping from the top of the screen to the bottom > Mobile Data, as this is required to send messages that exceed the SMS character limit, or contain images/emojis. If this is the case, you may wish to open the Messages app > Tap the 3 dots in the top right > Settings > More Settings > Multimedia Messages > Set Restrictions, and choose 'Warning'. This will notify you in the event that an SMS has converted to MMS format, in which case Mobile Data will be required to complete the sending process.
Now, go back to Messages > 3 Dots > Settings > More Settings > Text Messages, and make sure that a number is populated in the Message Centre section. If you can't see a number, contact your network directly to request the number, and manually enter it here.
If the issue is only affecting certain contacts, select the relevant contact/conversation thread in the Messages app, tap the 3 dots in the top right and make sure that they are not blocked.
To further troubleshoot the issue please open the Messages app > Tap the 3 dots in the top right > Settings > Chat Settings > Select Default Messaging Type, and set this to 'Text or Multimedia Messages'. Now, head to Settings > Connections > Sim Card Manager, and ensure that your preferred SIM is assigned to Messages.