Quit Photoshop if it is open currently.
Unable to open a photo in Photoshop with 'Edit In' command
On your Mac, when you select the Edit In Photoshop option in Lightroom, the photo doesn't open in Photoshop.
Lightroom and Photoshop
This issue occurs when you use Time Machine to back up Mac data on an external drive. The Time Machine function backs up both the data as well as the applications installed on your Mac resulting in two instances of Photoshop app files, which causes a conflict with the dynamic link between Lightroom and Photoshop.
For more details about Time Machine, refer to Apple's documentation: https://support.apple.com/en-us/HT201250
To resolve this issue, do the following:
Locate the disk(s) that you're using to back up your data with Time Machine.
Drag the disk(s) into Trash icon in the dock.
Once you've removed the disks backed up using Time machine, try using the Edit in Photoshop command in Lightroom now.