@nai8181: To troubleshoot the situation, please try enabling Mobile Data by swiping from the top of the screen to the bottom > Mobile Data, as this is may be required to send messages that exceed the SMS character limit, or contain images/emojis. Furthermore, head to Settings > Connections > Flight mode > Off, followed by Settings > Notifications > Do not disturb > Off. If you are currently abroad, please head to Settings > Connections > Mobile networks > Data roaming, and toggle this option on.
Now, if you're using the Samsung Messages app, 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. Tap the Back button > Push messages > On > Delete old messages > On.
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/multimedia messages'. Now, head to Settings > Connections > SIM manager, and ensure that your preferred SIM is assigned to Messages. I also recommend restarting your phone to address any temporary software issues that might be a contributing factor.
