This documents how to unlock a Monoprice motor desk frame 21590 21591 or 15722 (or similar.) It’s a great dual-motor height-adjustable desk frame, but Monoprice removed the manual and if you lost yours, you could be stuck.
On power-on, the desk control locked, and the display shows ---, and it seems like broken. However, it is most likely just locked.
Unlocking is as easy as holding a finger patiently on the ‘M’ for about 5 seconds until the --- disappears.