Fatal Error

I am having problems accessing the back end of the website. When i try to gain access to anything in the back end it appears with the following error code
Fatal error: Call to undefined function wp_doing_ajax() in /home/royalmedical/public_html/wp-content/plugins/advanced-access-manager/Application/Backend/Authorization.php on line 36

I believe this issue is due to the fact that the version of word press i am using (usnsure which exactly as i cannot access the back end) is not compatible with the newest update to the plug in

The page I need help with: [log in to see the link]