On your Mac, when you select the Edit In Photoshop option in Lightroom, the photo doesn't open in 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: