Hello Amitkumar Sirmagdum,
Thank you for reaching out to this Q&A Forum.
According to MS article "Graph functionality within SharePoint Online for GCC High is currently disabled. Any service that relies on Microsoft Graph may not currently be available" doesn't matter if you are using Graph explorer or any other tools.
It has been disabled for quite some time now, I am seeing it since last year.
Hope this helps.
Thanks.
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