Locate the WHDLoad.prefs file which is normally located in S:
It may be that the quit key is disabled by default, if it is simply un-comment out the line and change it to QUITKEY = $59. This maps it to the F10 key and not the print screen key (Which the A600 doesn’t have!)
Here’s an example of the RAW keycodes for a typical keyboard layout in case you wish to set something else, especially useful if you are emulating the Amiga on PC etc.