I am a Global Admin in Office 365, and Site Collection Administrator on a new site in SharePoint Online, so it’s pretty disconcerting to get the message: “Sorry, you don’t have access to this page” when configuring my site.
In Site Settings, Look and Feel, I clicked on Navigation. I made some changes to the Global Navigation, and clicked OK.
Of course, there is a box where I can request access. That message would be delivered to me. I tried that, just for fun. I got the request and approved myself. I tried again and still had the same error when I attempted to change the navigation settings in AreaNavigationSettings.aspx
I clicked on some other settings pages and realized I also could not configure the Site Output Caching or look at Popularity trends.
I worked with Microsoft to resolve this, sending them correlation IDs and Fiddler traces. Apparently this is a known issue happening in some tenancies.
First, connect to your SharePoint Admin URL in SharePoint Online via PowerShell (run as administrator)
Log in with your admin user account
Run the command: Set-SPOSite -Identity “https://<yourdomain>.sharepoint.com” -DenyAddAndCustomizePages $false
This PowerShell resolved the error in all of the site collections and according to Microsoft it will solve the problem in future site collections. Hooray!
Here is some further info from Technet on how to connect to your tenancy: