This plugin is invalid: The required file "start.php" is missing

I am maintaining a website running on Elgg 2.3.7. I have been trying to install a 3rd-party plugin called 'Widget Manager'. When I tried to activate this plugin, I got this error 'This plugin is invalid: The required file "start.php" is missing. Location: /var/www/html/mod/widget_manager/'.   The issue is that there is the start.php in this mod.

So I created an empty start.php file to fix this but it didn't solve the problem neither. The file permission for this plugin is the same as working plugins. Advice or solutions are appreciated. Thanks all!