Support the Get-AzureRmVmss Cmdlet
It seems the *-AzureRmVmss cmdlets are not supported in Automation. Specifically Get-AzureRmVmss, Update-AzureRmVmss and other cmdlets in the family. This is especially annoying since they are actually used in the "Scale Down Virtual Machine Scale Set Instances", "Scale Up Virtual Machine Scale Set Instances" Gallery runbooks.
Turns out we just had to upgrade the Compute module to 2.3.0: https://www.powershellgallery.com/packages/AzureRM.Compute/2.3.0
Who knows why a newly created Automation account would include an outdated module and would require a manual upgrade :-(