This error code 6004 will display on the printer's control panel when the connection between your computer and the printer might be lost.

Please follow the instruction to fix this issue.

1. Check the USB cable connection to make sure no damaged or broken.

2. Check the Canon official site to download the updated printer driver with the corresponding OS with your computer. 

3. To reset the printer by turning off the printer and unplug the power cord, wait for 15 seconds and replug the powder cord. Turn the power on of the the printer.