The classification of the expression and the element that it is made available for must match exactly.
in the expression editor, under the tab 'Availability for Classifications', you can select multiple classifications for availability, but of those classifications must match the one selected for the element under the properties section.
i.e. if you select the 'space' classification in the element, 'space' must also be selected (made available) in the expression. A general selection will not match a more specific selection , which may be your problem.
EDIT:
Also if you are using an expression within a expression, as the INT version is slightly different from the US, all expression classifications and the availability of all properties used must match that of the element.
Gerry
Windows 11 - Visual Studio 2022; ArchiCAD 27