If you're having trouble sending or receiving 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/receive messages that exceed the SMS character limit, or contain images/emojis. I'd also recommend checking Settings > Notifications > Do Not Disturb, and Settings > Connections > Flight Mode, and toggling these options off if they're enabled.
Now, open the Messages app > 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. Furthermore, enable 'Delete Old Messages' to ensure you have room to accommodate new messages.
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.