Hi! I understand you're having trouble sending SMS messages using the Nextplus app on your Mac. Let's figure this out together.

To start, could you tell me if the app shows any error messages when you try to send a text? Also, is this issue happening when you try to text certain contacts or everyone? This info will help me pinpoint the problem.

In the meantime, here are some initial steps to try on your Mac (OS X 10.15.7):

1. Check Internet Connection: Make sure your Mac has a stable internet connection since Nextplus needs it to send messages.

2. App Update: Ensure your Nextplus app is up to date. Sometimes, older versions can cause sending issues.

3. Blocked List: Sometimes, if a number is on your blocked list, messages won’t send. Double-check that the contact isn't accidentally blocked.

4. App Restart: Close the Nextplus app fully and then reopen it. Sometimes a simple restart clears glitches.

5. Log Out and In: Try logging out of your Nextplus account and then log back in.

6. Check Account Status: Occasionally accounts can get suspended or flagged, preventing texts. Verify your account is active.

If you have tried any of these already, let me know which ones, and if there’s any specific error message. That will help me suggest more advanced troubleshooting.

Are you able to share more details about the issue?