Error after installation

I have just added this plugin to an updated version of WordPress but it is not working, i get an error message

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in /home/{my server}admin_user_messages_settings.php on line 12

This appears on all pages created with the shortcode and also on the settings page.

Any support with this would be much appreciated

https://www.ads-software.com/plugins/admin-user-messages/