You configured your Robot to send hotkey and unlock the computer?
We must see what is actually a Robot. Robot is actually a Windows Service. Now lets see what is considered “Sleep”
When the computer enters sleep mode, power is cut to any unneeded systems, while RAM receives just enough power to enable it to maintain data. … Laptop computers often enter sleep mode when they are closed. A computer usually wakes from sleep mode through touching a key, clicking the mouse or pressing the power button.
So my guess is that the SERVICE (UiPath Robot) is not active, and that is why your Robot does not unlock your PC
NOTE this is only my guess, idk if it is accurate