Forbidden Error

I upload the zip file, choose lightbox, and put it into my page, but when I save and go check to see if it works, I get this message when I click on the launch button:

“Forbidden

You don’t have permission to access “(articulate file)” on this server.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.”

The lightbox comes up, but the file doesn’t launch. My knowledge of website building is extremely limited, so for all I know this is a really easy fix, but I have been trying for hours and I can’t figure out what’s wrong.
I thought maybe it was because I’m using gantry5 to build my website, but even when I load it on the twentyfifteen theme and deactivate every unnecessary plugin, I still get the same error message.

Please help!

https://wordpress.org/plugins/insert-or-embed-articulate-content-into-wordpress/