You have specified that sloped pipe be drawn, but are trying to connect to an element that doesn't support sloping connections.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.4.0.0 (26.4.0.0)
Syntax
C#
public static FailureDefinitionId AttemptToConnectNonSlopingElementToSlopedPipeError { get; }