You have selected elements that are already part of other Circuits that have assigned Panels. The elements you selected cannot be added to this Circuit.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.2.0.0 (26.2.0.0)
Syntax
C#
public static FailureDefinitionId ElementsBelongToOtherCircuit { get; }