Plugin not installing on wordpress


I have recently become your paid member.

I tried to install- Add New User- plug in on WORDPRESS gave same message when I tried to activate it

Fatal error: Call to undefined function is_site_admin() in /home1/taxworry/public_html/wp-content/plugins/NewUsers/update-notifications.php on line 417

This also happened when I tried another plugin pop-Over plugin.

Please do the needful.