Usually you can get past this error by quitting Microsoft Word. To do this:
1. Command-Tab to switch to Microsoft Word.
2. Command-Q to Quit Microsoft Word.
3. Start Microsoft Word, but make sure there are no prompts waiting for your attention in Word.
Now go back to TntMPD and try to launch the mail merge again.
If this still fails, you may need to perform a mail merge manually. Click here to learn how to manually perform a mail merge.