It is just what the message says. You are asking the machine to home to a homeswitch. The machine looks at the homeswitch, buts says "the switch is ON so I must be there" . One assumes you either have home switches fitted, or you have fitted them yourself, so the switch you are asking the machine to home to is already on.
If you look at the Diagnostics page, you will see the home switch LED is lit ( usually yellow). Find it in the ports and pins section and look at Active low. If it is off, the tick it, if it is ticked then untick it. Look back at the Diagnostics page and the LED should have gone out. THERE SHOULD BE NO LEDS LIT WHILST THE MACHINE IS RUNNING.
It cannot be doubled with a limit switch or the "E stop" would keep firing.