Split does not work on curtain walls containing mullions created in Revit prior to version 4.0. You may delete the mullions, place them again, and then proceed with splitting the wall.

public static FailureDefinitionId CannotSplitOldMullions { get; }

Public Shared ReadOnly Property CannotSplitOldMullions As FailureDefinitionId
	Get

public:
static property FailureDefinitionId^ CannotSplitOldMullions {
	FailureDefinitionId^ get ();
}