There is not sufficient info to determine a problem, but you might check the following:
1. The open and closing () must match -- you are missing a closing )
2. Strings are case sensitive unless you select otherwise.
3. Your classifications must match for both the expression and the element
4. Check that the property 'Build Material' is available for the element in question - its not available for walls. A easy way to check is to enter only the property with the correct data type 'string' -- and use the evaluate button -- look for a error message.
5. What element type are you checking? -- if I know that I could double check your formula
6. Properties must be entered via the dropdown icon not directly typed in. Check that their highlighted in gray?
7. Be sure your separator for your version is ; and not ,
Gerry
Windows 11 - Visual Studio 2022; ArchiCAD 27