Check the following points to resolve the issue:

• The paper size specified in the printer driver and the machine's paper size setting are not suitable for automatic 2-sided printing.
  Press Stop/Exit or (Cancel). Select a paper size suitable for 2-sided printing.
  The paper size suitable for automatic 2-sided printing are A4, Letter, Legal, Mexico Legal, India Legal and Folio^*.
   
• The paper in the tray is not the correct size and is not suitable for automatic 2-sided printing.
  Load the correct size of paper in the tray and set the tray for that paper size.
  The paper sizes suitable for automatic 2-sided printing are A4, Letter, Legal, Mexico Legal, India Legal and Folio^*.

*: The paper size for automatic 2-sided printing may differ depending on your country or region.