The Find Element functions in ATS are mapping to the native selenium functionality. They are intended to be used when the standard mendix functions are not applicable.
In your case in order to find a widget you can use the "Find/Assert widget" function - https://docs.mendix.com/ats/refguide/rg-version-2/function-reference#5-widget-find
Quoting from the documentation:
Finds/asserts a Mendix widget by its given name. It is possible to use a sequence of names as a path (separated by spaces).
I would suggest to use this function to find a widget in a mendix up. You only need to pass the mendix name e.g. 'container123'.
If you want to still use "Find element by CSS" then you need to add a dot at the begging of the class
Hope this helps,
PS: The link you posted was from the old reference guide for ATS 1. I am not sure if you are using ATS 1 or ATS 2, but please check if you are looking in the right reference guide.