Hi @Amit Rawat ,
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:
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:
- Go to Central Administration.
- Click Security and then select Configure Managed Accounts.
- The list of existing managed accounts is displayed.
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