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Based on the error message it seems the issues with authentication or authorization of your user account on the Azure Key Vault.
Could you please check the user account you're using in Power Automate has the necessary permissions to access the Azure Key Vault.
You can grant permissions to your user account by assigning a role to it. In Azure Key Vault, you can assign roles to users, groups, and applications to control their access to the Key Vault.
To assign a role to your user account, you can read & follow: Provide access to Key Vault keys, certificates, and secrets with an Azure role-based access control
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Akhilesh.
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