That's an old cmdlet, likely the module version you downloaded does not contain it. Try using the Set-AIPFileLabel cmdlet instead: https://learn.microsoft.com/en-us/powershell/module/azureinformationprotection/set-aipfilelabel?view=azureipps
AD RMS file bulk decryption using PowerShell
We’re in the process of trying to do a bulk decryption of Office files that were encrypted by AD RMS. We've identified a potentially effective PowerShell command, "Unprotect-RMSFile," that could address our decryption needs. We expected this command to be included with the Azure Information Protection Client or the AzureInformation Protection PowerShell module as Microsoft documentation indicates it but it appears to be missing as seen in the screenshot below when we tried to test it. Can you help us find a way or let us know what is the module required to run the command and its requirements?