according to this wiki:-
shutter should work inside whonix. but i dont think this is the same with Whonix inside Qubes OS, and i think the cause of that is qubes security itself “compartmentalization”.
shutter doesnt recognize or allowed to view whonix Desktop. so it is giving this message:-
[glow=red,2,300]Error while taking the screenshot.
Keyboard could not be grabbed[/glow]
and also another Error appears if u press on Menu inside shutter program
[glow=red,2,300]Error while taking the screenshot.