Got Forbidden message

I tested download links on my development, everything works fine.

However, when I use this plugin to create plugins on the server. I got this message.

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Forbidden

You don’t have permission to access /wp-content/plugins/download-attachments/includes/download.php on this server.
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Here is the site link:
http://tjcorg.staging.wpengine.com/tjc-demo-site/feature/file-download/

I assume this is something related to the .htaccess settings. How do I fix it?

https://wordpress.org/plugins/download-attachments/