It's weird. I have logged in using the Lightroom app on both my iPhone and my iPad and I can sync photos between those two devices. But not in Lightroom on my Mac.
I tried logging out of Lightroom/Creative Cloud on my Mac and then logged back it, but that didn't change anything.
Hm. Figured it out.
Under the nameplate, "Sync with Lightroom mobile" was turned on. I toggled it off, then back on again, and the boxes magically appeared.
Frustrating, to say the least, but it worked.