Curtain grid lines cannot be copied. If you are trying to copy the curtain wall, try using tab to select entire wall.
Namespace: Autodesk.Revit.DB
Assembly: RevitAPI (in RevitAPI.dll) Version: 26.4.0.0 (26.4.0.0)
Syntax
C#
public static FailureDefinitionId CannotCopyGridLinesWarn { get; }