Please check the following points:

• Change to a recommended type of paper, and then check that the problem is solved.
  To see what is the recommended paper, click here.
  To see what kind of paper you can use, click here.
• Check that the back cover is closed properly.
• Check that the transfer roller is installed properly.
• Check that the fusing unit is installed properly.
• Replace the fusing unit.
  To see how to replace the fusing unit, click here.