WP File Download plugin is not installed correctly
WP File Download uses its own framework which is automatically installed in the /wp-content/mu-plugins
Sometimes this directory has wrong permissions and WP File Download can't install its framework.
To fix this issue :
- Create the folder /wp-content/mu-plugins
- Make sure the /wp-content/mu-plugins is writable by your webserver user
- Go again to your wordpress administration page and check if you still have the error.
You still have the issue
- Unzip the content of wp-file-download.zip file on your local machine
- Create the folder /wp-content/mu-plugins on your website if it doesn't exist
- Copy the content of the /wp-file-download/framework/ folder from the previously unzip files to the /wp-content/mu-plugins/ folder in your website server
- Go again to your wordpress administration page, you should not have the error anymore