P: View > Sort > Last Comment Time incorrectly greyed out

I had the same issue and this was the first result from a Google search.
For anyone else finding this you can still sort by last comment by going to:
Library -> Review Comments

"Why isn't likes and comments a Smart Collection?"

I see that you already voted on that topic, good: https://feedback.photoshop.com/photoshop_family/topics/lightroom-better-support-for-comments-and-lik...

I'm having that greyed out menu item as well. The bottom tool bar does work.

Why isn't likes and comments a Smart Collection?

That's too bad you couldn't get it to work.  But at least we have two concrete, reproducible bug reports filed.

I give up, the reset not worked, then I decided lost all comments that I have, and use the option to delete all data  in Lightroom Mobile, this work out.

I lost the olds comment, but all working again

Tanks for help

I will try Reseting the preferences

The option View > View Options > Thumbnail Badges was checked. I had created a new collection with photos that hadn't been edited. 

Now, I edit some.

Those screenshots should be very helpful to Adobe. 

Regarding the lack of the comment badge in the lower-left corner of the thumbnails, it looks like all badges are missing, including the badge that indicates a photo is in a collection.   Verify that the option View > View Options > Thumbnail Badges is checked:



If that doesn't help, try resetting LR's preferences: http://www.lightroomforums.net/showthread.php?25197-Resetting-the-lightroom-preferences-file-updated. LR sometimes soils its preferences, causing the user interface to go weird, and resetting often fixes things.