"Sorry, could not reserve the appointment… Please try again later."

Sorry about the problems.

I made updates to the file –> _include/_form_confirm.php

I think it should work now. I can’t duplicate the error on my system, but I am pretty sure I know where the problem is originating.

Try and reinstall the plugin or you can get the new file here:
http://www.skedmaker.com/_form_confirm.zip

unzip it and replace it with the one in your directory.
I hope this helps, please let me know if not.