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 22.214.171.124 or later. For instructions, see Installing the AADRM PowerShell module.