Hi there! I see you're having trouble sending a message with the textPlus app on your iPhone running iOS 18.3. Let's get that sorted out together.

First, just to understand better, when you say your message is not sending, do you see any error messages or indicators? Also, is this happening with all your messages or only when messaging specific contacts?

Meanwhile, here are some simple steps you can try:

1. Retry Sending: Often, simply tapping the failed message to resend can work if there was a temporary glitch.

2. Check Recipient Number: Ensure the person you're texting can receive SMS messages and that the number is correct.

3. Restart the App: Close the textPlus app completely and then reopen it.

4. Check Your Assigned Number: Log out of textPlus and log back in to confirm your account still has an active assigned number.

5. Internet Connection: Since textPlus uses the internet for messaging, make sure your iPhone has a stable Wi-Fi or cellular data connection.

6. App Permissions: Verify in your iPhone settings that textPlus has permission to use cellular data and notifications.

If none of these help, sometimes clearing temporary data can assist, but since you're on iOS, that usually means uninstalling and reinstalling the app rather than clearing cache manually.

Would you be comfortable trying these steps? Also, please let me know if you see any particular error messages or if you have already tried some of these fixes. That way, I can guide you further.

For reference, the official textPlus help center article on resolving message sending issues is a helpful resource too1.

Let me know how it goes!