2-Sided Printing (Windows) or Two-Sided Printing Settings (Mac) is gray or not available for selection. What should I do?

• Automatic double-sided printing is only available if you have loaded plain paper or other supported paper in these sizes: Letter (8.5 x 11 inches [216 x 279 mm]) and A4 (8.3 x 11.7 inches [210 x 297 mm]).
  

  Note: Double-sided printing is not supported for printing on glossy, thick, and certain matte papers.