Hi himani ghildiyal:
Thanks for reaching out to Microsoft Q&A.
You should have Key Vault Data Access Administrator, User Access Administrator or Owner permissions to change access configuration policy. Could you double check that?
Changing permission model requires 'Microsoft.Authorization/roleAssignments/write' permission, which is part of Owner and User Access Administrator roles. Classic subscription administrator roles like 'Service Administrator' and 'Co-Administrator' are not supported.
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