Questions about display

This pic means that why the files do not show in the main window? It just shows me no selection.
I think it should show the documents when I click the file on the left side. And Devonthink2 has not the similar problem.
So, am I not set correctly?

DEVONthink 3 does not show any files in the view/edit pane unless you’ve enabled a hidden preference.

Open Help > Documentation > Appendix > Hidden Preferences and click the On link for DisplayGroupsInPreviewPane

Thanks, I’ve set it.

However, why cannot display groups but only display files?
It means that Devonthink 3 has canceled this function? …

That is correct. As noted in the Help, DEVONthink will only display files in this mode, not groups.

Does it only display as icons, or am I missing how to switch this view?

Yes, it only displays as icons. It is not the item list, so there are no alternative views for it.