Hi @Hari Vidya Sankar ,
You need to have "User Access Administrator" or "Owner" Permissions to assign specific roles to a resource as these roles has Microsoft.Authorization/roleAssignments/write permissions which normally Global Administrator doesn't have. GA can mange all aspects of Azure AD but managing resources you need to other RBAC permissions. Here is the documentation for your reference.
Please let me know if you have any other questions.
Thanks
Saurabh
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