403 Forbidden

Hello! The calendar displays normally on the webpage. However, when I try to schedule an appointment by pressing the “schedule an appointment” button, I get this error message instead of the calendar form:
Forbidden

You don’t have permission to access /wp/wp-content/plugins/appointment-calendar/calendar/calendar_form.php on this server.
Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

Any reasons why?

The page I need help with: [log in to see the link]