The phases of the two surfaces you are merging do not overlap. You can continue the operation, or cancel it and select a different pair of surfaces.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.3.0.0 (26.3.0.0)
Syntax
C#
public static FailureDefinitionId SitePhasesDontOverlap { get; }