In case anyone else encounters this:
I recently deleted Photoshop beta from my computer because the new AI features are in the stable version. I think that's what caused this problem. the next time I went to the "edit in" menu in Lightroom, photoshop was grayed out, and when I tried to put it back using preferences, there were no options in the drop-down box. LR simply didn't recognize that Photoshop was there.
The online solution is to uninstall and reinstall both Lightroom and Photoshop. However, it seemed to me that uninstalling and reinstalling only photoshop might do it, and indeed, that fixed the problem.