@Sameer Bhat Welcome to Microsoft Q&A Forum, Thank you for posting your query here!
Based on the error message Add-AzKeyVaultManagedStorageAccount : Operation returned an invalid status code 'Forbidden' there is a similar thread discussion, please refer to the troubleshooting steps and let me know the status.
Also check in the access policy->Select principal is been added
You can also refer to this thread it explains How RBAC works and designed.
You can provide or
You can check this article for a complete example: Add-AzureKeyVaultManagedStorageAccount
Please let us know if you have any further queries. I’m happy to assist you further.
Looking forward for your reply!
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