The mullion(s) you are trying to place is too short for his type, and was not placed on the grid.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.3.0.0 (26.3.0.0)
Syntax
C#
public static FailureDefinitionId AllMullionsAreTooShort { get; }