The PIXMA MG5220 must have all of the inks installed and contain ink for the unit to able to print.
If you are experiencing a "Low Ink" warning, you will still be able to scan, but not print.
If you are getting a "Ink Out" error, you will not be able to use the unit until the ink is replaced.
These precautions are in place to prevent damage to the printer from occurring if printing with no ink is attempted. The printer uses the ink to cool the printhead during the printing process. If no ink is present, the printhead could be damaged.