Failed opening required

WordPress includes many functions for determining paths and URLs to files or directories within plugins, themes, and WordPress itself. 

 $plugin_dir = plugin_dir_path( __DIR__ ); // wp-content/plugins/
 require_once($plugin_dir.'WPNonce/WPNonce.php');

While using plugin_dir_path(), keep in mind that the “plugin” part of the name is misleading – it can be used for any file, and will not return the directory of a plugin unless you call it within a file in the plugin’s base directory.

or alternatively, you can also use WordPress constant WP_PLUGIN_DIR

 require_once(WP_PLUGIN_DIR.'/WPNonce/WPNonce.php');

I hope this helps.

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