Hello @Pankaj Mandrawal ,
Please make sure you have enabled creation of subsites for sites in SharePoint Admin Center: https://learn.microsoft.com/en-us/sharepoint/manage-site-creation#manage-detailed-site-and-subsite-creation-settings-in-the-classic-sharepoint-admin-center
Currently, you cannot create sites/subsites using graph API. But you can read by using the following API:
GET /sites/{site-id}/sites
Hope this helps.
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