Please check the following points to solve the problem:

NOTE: Illustrations shown below are from a representative product and may differ from your Brother machine.

• The back cover of the machine is not closed completely. Close the back cover of the machine.
  
  
  
  
  
• The duplex tray is not installed or not installed correctly. Re-install the duplex tray. Duplex tray is located in the rear of the Brother machine.