Smart Collection based on another Smart Collection

Deleted.

A 'Smart Collection' isn't considered a valid source.

One way to think about this is that a Collection is a "thing" - a noun (a defined set of photos), while a Smart Collection is more like a verb: (apply some selection rules and keep doing that from moment to moment). 

If you want a Smart Collection which adds a further selection rule to modify those results: Duplicate the first Smart Collection and then add this extra rule to that. 

Noting, if your first Smart Collection has "Any of the rules" logic [where each new rule widens the set], and your extra condition needs "All of the rules" logic [where each new rule narrows the set] - you may need to redesign those first rules accordingly within this duplicate.

For example: if current SC's rules are Any of [keywords include "france"] OR [keywords include "italy"] and what you now want is to add the requirement to be 3 stars or over, you might need to set the rules of this new SC as All of [keywords must include "france italy"] AND [stars must be 3 or more] .

The space between those keywords is in effect 'smuggling' some OR behaviour inside the AND requirement.

The Collection criteria in your filter has to be a simple Collection, not a Smart Collection.  This will not work.