What language are you using? That message is basically saying that the COM_JFBCONNECT_NEW_COMMENT_BODY string isn't defined in your langauge file. If you're using a language other than English, I'd recommend you compare it against the /language/en_GB/en_GB.com_jfbconnect.ini file and copy/translate any strings missing to your language file.
Feel free to post any updates you have. We'd love to include more language files in our releases, or update any issues you may find with the language files we do include.
Thanks,
Alex