Additional features, settings, or issues not covered by specific Microsoft Teams categories
As mentioned in the documentation, you need the PREVIEW module for that parameter:
If you're using the PowerShell steps in this article to manage private channels, you must install and use the Teams PowerShell public preview module from the PowerShell Gallery. For steps on how to install the module, see Install Microsoft Teams PowerShell. The latest General Availability Teams PowerShell module doesn't support managing private channels.
Get it here: https://www.powershellgallery.com/packages/MicrosoftTeams/1.1.3-preview