You did install it, but did you import it to your PS-session?
Check your current session using Get-Module
And if not loaded, simply: Import-Module MicrosoftTeams
Seems like the cmdlt
Get-TeamChannelUser is still in development and are only included in the public preview version.
However, you should be able to install the latest public preview version.
Install Teams PowerShell public preview
If you're using the Public Preview version of Teams PowerShell, we
strongly recommend that you first uninstall Skype for Business Online
Installing the Teams PowerShell public preview module for all users on
a system requires elevated privileges. Start the PowerShell session
using Run as administrator in Windows or use the sudo command on macOS
If you're using PowerShell 5.1, you must update the PowerShellGet
module beforehand. After you update PowerShellGet, close and reopen an
elevated PowerShell session to ensure that the latest PowerShellGet is
Install-Module PowerShellGet -Force -AllowClobber
To install Teams PowerShell public preview, run the PowerShell command below.
You can find the latest preview version at PowerShell Gallery or in
PowerShell by running "Find-Module MicrosoftTeams -AllowPrerelease"
Install-Module MicrosoftTeams -AllowPrerelease -RequiredVersion "1.1.3-preview"
(1.1.5) is the latest preview version as for 2020-09-16).