What does it mean with the LCD screen on my CO alarm displays the letters ERR?

An ERR message indicates an alarm malfunction. It will alert you with a short "chirp" and the red LED will flash once every 30 seconds. Push the Test/Reset button; hold it for 5 seconds, then release. If the reset is successful, the display will reset to zero. If it's an actual malfunction, the unit will go back to ERR within 5 minutes. The alarm should be replaced.