Sometimes the electrical switch that checks if the front panel is in place doesn't recognize that the panel is there (see picture #1 for the switch under the control panel, and the bolt on the front panel that triggers the switch). This can be fixed by pushing a little harder on the front panel where that bolt is located while turning on the unit (see picture 2).
Picture 1 (looking down from the top of the unit with the top grate removed to expose the electronics. That silver bolt I am pointing to is attached to the front panel that you take off to check the filters)
Picture 2 (press harder where my thumb is - about straight down from the on button to get that switch to recognize that the panel is in place)