If, in your expression, you do not see the yellow warning icon, then your expression itself is valid, and the problem is either with a conflict in the classification settings or the availability of a properties you are using.
You have not given sufficient info to determine which. A few screen shots would help.
However in expressions not all the properties listed for stairs are available for all types of stair construction. Most under just the main stairs group are, most under the stair structure group are not.
You just have to check them individually to see. Perhaps Archicad TS could help -- The help site is no help as to which are or are not.
There is a net volume property under general which may give you what you want. otherwise you could calculate the stair volume but nothing for runners as I could see?
Gerry
Windows 11 - Visual Studio 2022; ArchiCAD 27