This bug seems to have crept in with Sierra, though I saw isolated incidents with whichever bit of California topography preceded Sierra. Sometimes a restart of Scrivener (I also see it in Mail and TextEdit) fixes the problem; sometimes I need to restart the computer. In all cases, it’s not that selected text can’t be moved; it’s that the cursor activity takes an inordinate length of time to change from Selecting to Moving. Keep holding down the trackpad or mouse button or ever how you select, and the cursor will eventually (8 or 10 seconds sometimes) change and you can move your block of text.
It’s irritating, but workable enough that I don’t bother doing a restart in the middle of a work session.