Sorry that Lightroom does not open Photoshop using the Edit in command, instead it creates a new tiff file next to the file you're working on in Lightroom.
Could you please take a look at this article Unable to open a photo in Photoshop with 'Edit In' command in Lightroom and let us know if that helps?
That solution looks promising inasmuch as the problem is occurring on a new iMac and I just migrated photos and apps to it using the migration assistant and my Time Machine disk. However, the article says "Locate the disk(s) that you're using to back up your data with Time Machine" then "Drag the disk(s) into Trash icon in the dock." In my case, the external disk I'm using to back up data with Time Machine is not presently connected to my computer, and there is no disk to drag to the Trash icon.
The other thing is, the photos I am attempting to open are photos I took after the migration and they were never part of my Time Machine backup.
I plugged in the disk used for Time Machine and opened LR. I tried to do an "Edit in Photoshop" and it worked. I ejected the Time Machine disk and LR continues to function properly. I have no idea what happened, but everything is working again (for now!).
Thank you for your time and assistance.
It seems the Time Machine have a non working state of Lightroom (be it the files or the catalog itself) that is causing this issue. Thanks for the heads up, glad that you got things back to work.