This is not new. I have been through it before but don't remember how did i fix it. May be you can help me !!

Though you are site administrator (full control) but many of the administrative functions repeatedly (3x) prompt you for the authentication credentials and then give out the Access Denied error message.

For example going to Site Administration Page (SUB)SITE-URL/_layouts/1033/webadmin.aspx and then under Users and Permissions to Manage Access Request.

Access denied. You do not have permission to perform this action or access this resource.

Access requests are not enabled.

I have tried Knowledge Base and could find any article to fix this. Earlier (when I had this problem else where) I was able to locate an article and follow it through. Fix was something to do with ananomus access and Auth setting on Portal and IIS.