question

Hi @AmitRawat-3636,

To get SPSite.Usage.Storage data, the user need to have full control permission for the site.

Could it that one user is the site collection administrator or he has full control to the site?

Please refer to this article for more:

https://docs.microsoft.com/en-us/SharePoint/sites/user-permissions-and-permission-levels.
------------Update-----------
Also make sure the user is a managed account, to check if a user is managed account or not, you can refer to the following:
1. Go to Central Administration.
2. Click Security and then select Configure Managed Accounts.
3. The list of existing managed accounts is displayed.

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                     foreach (SPWeb subSite in sps.RootWeb.Webs)
                     {
                         if (subSite != null)
                         {
                             //Do Something
                         }
                     }
                        
                 }
             }
         });

how exactly do you run provided code? Is it running with Sharepoint context (e.g. as SP web service or application layout page opened in context of some SP site like this http://example.com/_layouts/15/MyPage.aspx) or you run it without SP context (e.g. as console application or SP job)?

• If you run it with SP context then you need to modify your code like that:

  SPSecurity.RunWithElevatedPrivileges(() =>
  {
  using (var elevatedSite = new SPSite(webApplicationUrl))
  {
  var webApplication = elevatedSite.WebApplication;
  foreach (SPSite sps in webApplication.Sites)
  {
  string size = FormatSize(sps.Usage.Storage);
  }
  }
  });

I.e. it is necessary to reopen SPSite under RunWithElevatedPrivileges - otherwise RunWithElevatedPrivileges won't have effect. After that code will run under SHAREPOINT\System account (=account of app pool used by Sharepoint web site in IIS) which should have access to sps.Usage.Storage.

• If you run this code without SP context - then you don't need RunWithElevatedPrivileges since it has effect only when SP context presents. In this case code is always running under account of user which ran the code. If it fails and you already added Full control policy for this user on this web application, try to add this user to Site collection administrators of each site collection in web app including root site collection (go to Site settings > Site collection administrators > Add user under which you run this code. Repeat this step for all site collections in web app)

                     }
                 }
             });