This cmdlet is now removed from the latest version of the Azure Rights Management module, because the Azure Rights Management service uses Azure Key Vault for customer-managed keys.
Instead, use Use-AadrmKeyVaultKey. For more information, see Planning and implementing your Azure Information Protection tenant key.
In addition, update your version of the module so it is 184.108.40.206 or later. For instructions, see Installing Windows PowerShell for Azure Rights Management.