The toner cartridge or the drum unit and toner cartridge assembly is not installed correctly.
Pull out the drum unit. Take out the toner cartridge, and then put it back into the drum unit following the steps below:

1. Make sure that the machine is turned on. Leave the machine for at least 10 minutes to cool down.
2. Press the front cover release button and then open the front cover.
   
   
   
    
   
   
3. Take out the drum unit and toner cartridge assembly.
   
   
   
   
   
4. Push down the green lock lever and take the toner cartridge out of the drum unit.
   
   

5. Put the toner cartridge firmly into the drum unit until you hear it lock into place. If you put it inproperly, the green lock lever on the drum unit will lift automatically.
   
   

6. Put the drum unit and toner cartridge assembly back into the machine.
   
   

   
   

7. Close the front cover of the machine.