I get this error when I run your PS against a virtual switch that already has the security settings extension installed. Try your PS with Set-VMSwitchExtensionPortFeature instead of Add-...
If that still doesn't work, first run
Get-VMSwitchExtensionPortFeature -ExternalPort
If that shows the security settings feature and nothing else, then you can:
Get-VMSwitchExtensionPortFeature -ExternalPort | Remove-VMSwitchExtensionPortFeature
If you have more than one feature installed, then you'll have to add filters to the Get-VMSwitchExtensionPortFeature until you narrow it down to the one that you want.
After you have successfully removed the feature, you should have no trouble with your original PS that uses the Add-...