Canon as well as other manufacturers do this for two reasons. One is for warranty purposes. The other is to help ensure consistent results.
When that message appears, you can continue printing but, the print quality can deteriorate. With the message, you can press the online button to continue printing. If you are still unable to print, I would suggest contacting support again at 1-800-652-2666 for assistance with the toner issue.
We would be happy to further assist with the issue via chat. To provide some feedback or some troubleshooting steps, we would need some additional information. If you could reply to this message with the information below, we can provide some assistance.