Curtain element Panel doesn't belong to Curtain element. In merging, you must select the version of a Curtain element that has had Panels added to it.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.3.0.0 (26.3.0.0)
Syntax
C#
public static FailureDefinitionId CurtainPanelNotBelongsToElement { get; }