You don't have permission to access that file.
That likely means that you need to log into your server and change the permissions on that file. If it's Linux, and you can get to command-line, just use chmod
:
chmod 774 /path/to/wp-admin/admin-ajax.php
If it's some other configuration, you may need to ask another question, or update this one with those details if you can't figure it out. A question like that may actually be better suited for StackOverflow's sister site, StackExchange.