Grouping is greyed out

I'm new to Opus and trying to get up to speed on basics. I want to do a flat view of a disk drive then group by file extension, but after I click on flat view the options for "Group By" are greyed out. Is there a way around this?

You can group in two of the Flat View modes: Mixed and Mixed (No Folders)

Both modes display everything in a single, flat list, so grouping can be applied on top of it if desired.

The other mode, Flat View: Grouped already groups everything by folder, so you can't group things when in that mode.