Endstops tell the printer when the travel limit has been reached on a particular axis. The Duet's connections are a little different than RAMPS style boards, so please review this step and your endstops carefully.

X Endstop

Y Endstop

Z Endstop

From left to right on an endstop connector, the pins are signal, +3.3v, and GND.

If using 2 wire microswitch endstops, connect one wire to GND and the other wire to signal (the outer two pins).

If using other endstops, please refer to this link for details.

Never connect your endstop wires from +3.3v to ground. This will create a short circuit and could damage the Duet.