The Paper Size setting does not match the size of paper in the tray. Or, you did not set the paper guides in the tray to the indicators for the paper size you are using.
To solve the problem follow the steps below:

1. Check that the paper size you choose matches the size of paper in the tray.
   
   
2. Make sure you loaded the paper in a Portrait position, by setting the paper guides at the indicators for your paper size.
   
   
3. After you have checked the paper size and the paper position, press Black (Mono) Start or Color (Colour) Start to continue printing.