Installed Plugins saying "Access Denied"

I am helping create a website that is currently private and I have installed some plugins that I need to deactivate, but the “Installed Plugins” tab is not appearing under Plugins. When I click on Plugins itself, it takes me to the “Add New” plugin page. I tried typing in the URL (/plugins.php) and it says “Access Denied.” I am listed as Administrator on the website so I shouldn’t have a problem with Access.

I had the owner go in the back end and delete the plugins and we still can’t see the “Installed Plugins” tab.

Thank you for your help in advance!