Do I need an AD-E1 adapter on my R6 Mark II for my Speed lite 600 RT II?

If it doesn't work, make sure that you are not using electronic shutter mode. It's easy to forget about this limitation if you are only using flash sparingly.
Happens to me sometimes. The R3 does support flash with electronic shutter, but not in silent shutter mode. I use silent mode sufficiently often that I have the toggle assigned to the video/stills switch.
 
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