I am using latest build of PM6.
This only seems to work if there are at least two photos already selected in the contact sheet.
I tried with no photos selected, both by opening Preview by clicking on the mag glass icon, and by double-clicking the photo.
The command is disabled in the toolbar, also the key shortcut does nothing.
Similarly if I select one photo and open preview, the command is disabled, and remains disabled if I transit to another photo in the filmstrip in Preview.
This is puzzling behavior. I'm used to just culling in Lightroom by using the pick and delete keys. Would like to move the culling process upstream sometimes to PM, for instance when shooting birds, I have a pretty large collection of non-keepers
Just noticed the behavior is even stranger: I have to open a photo in Preview, then make a selection in contact strip to enable the command !!!!!