“Sorry, you don’t have access to this page” to Site Permissions page in SharePoint 2013

If the site owner, who supposed to have full control, lost the permission to access the Site Permissions page, while everything else looks fine. The following could be a fix.

“Access Denied” to Access Requests list or “Request approval failed” when you process a pending request in SharePoint Online

Basically you have to reset the permissions to the Access Requests page, which is the root cause. You will need Site Collection Admin permission to make this fix, and you will following the article above to find out the listid of the Access Requests list.