This means that the physical download file does not exist.

This is usually caused because the folder that contains the download file was not created. Try deactivating the plugin, count to five, then reactivate the plugin. If that does not work then the most likely cause is that the permission on your host server are set high. If this is the case you will then need to manually create the download folder. 

Using your FTP program or using your Website administrative panel ( Cpanel, Plesk etc. ) go to the "wp-content/uploads" folder. Create a new folder called "doifd_downloads" Then change the permissions on the uploads folder to 755.