You have selected elements that are already part of other Systems that have assigned Equipment. The elements you selected cannot be added to this System.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.4.0.0 (26.4.0.0)
Syntax
C#
public static FailureDefinitionId ElementsBelongToOtherSystem { get; }