Answer: please see the following

This is the message to warn you to check on the cartridge to see if it is empty; and it appears occasionally. Remove the washing liquid cartridge and check it’s not empty; and reinstall the cartridge to reset the counter and it will cancel the message.

* When OK is selected with no washing liquid cartridge removal, then it will cancel the
　message at the moment, but it will appear once again when you restart the printer.