Having banged my head against this misleading error message, I thought I’d post this.

it’s nothing to do with any of the parameters you’re passing in, believe me I checked.

It’s actually complaining that the Credential already exists! I was doing a redeployment and the credential hadn’t been removed.

And in other useful annoyances, Find-AzureRmResource doesn’t work for Automation Credentials. So now my deployment scripts looks like this:

if ((Get-AzureRmAutomationCredential -ResourceGroupName $rgname  -name "meadmin" -AutomationAccountName $AutoName)){
    Remove-AzureRmAutomationCredential -ResourceGroupName $rgname  -name "meadmin" -AutomationAccountName $AutoName 
  }

New-AzureRmAutomationCredential -AutomationAccountName $autoname -ResourceGroupName $rgname -name "meadmin" -Value $acred -Verbose