In the item Type window when any expression needs to add an error message popped up. And the required expression cannot be entered. This issue only occurs in Update 8. So, here is the way how to add any expression without having the error message.
Step-1, Go to this location, C:\Program Files\Bentley\OpenBuildings CONNECTEdition\OpenBuildingsDesigner\expressionbuildertoken.fa.dat And delete this file. Step-2, Restart open building designer. Then Go to Item Type from Data/Reporting ribbon, Step-3, Now you can manually write the expression in the dialogue box. (here, for example, we need an expression for the Opening area. So, our expression is this. Height * this. Width)