Frequent Error Message on Sending Messages

*sigh* Have you changed the jos-prefix according to your Joomla setup?
No? So, you have *not* applied the changed from post #212882 successfully.

No I have not done this. Please understand that I'm a knowledgeable Joomla user but NOT a techie. I'm pretty good at following instructions though. If you or someone else can tell me how to complete this step, I will do it or get someone to do it. Much appreciated.