How to Clear the "Unable to Print" Message from the Brother Printer Display
Found this on youtube. If this doesn't work, try the one below.
Access the internal maintenance mode by pressing the Menu/Set button, then press * 2 8 6 4 within 2 seconds. This should put you in the maintenance mode menu. Enter 80, then scroll down with the arrow down (round) key. You'll find an entry listed as purge count. If number is over 6000, enter 2 7 8 3.
This will reset the purge count value. Then the printerwill restart, hopefully without the annoying cannot clean message.