Can't create Corner Mullion due to the angle between the neighboring panels. Try changing the Parameters in the Curtain Wall Mullion Family.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.3.0.0 (26.3.0.0)
Syntax
C#
public static FailureDefinitionId CannotCreateCornerMullionDueAngle { get; }