Are you using Dropbox to sync the catalog from IMac to MacBook & visaversa ?
It is possible you have different versions of the catalog for each computer and the Photoshop file is only being added to the catalog that is currently active. Catalogs must reside on a fixed internal drive or a connected external drive.
If you move the external drive between computers with all your image files, you would be better to have the one catalog on that same drive, so it 'travels' with the photos, and no possibility of syncing problems.
Also, could you please confirm the Lightroom version you use?
Do you face this issue on both iMac and MacBook pro?
What if you create a local catalog, not as a part of Dropbox, and try to do "Edit in PS" ? Does it work as expected?
I use 2015.8 on both computers.
The issue is on both computers.
Did create a new catalog and imported the old one and same problem.
Tried with new catalog and imported one photo and after editing in Photoshop 2017 it comes back with an empty image with "!" and the following message: “LaCie:Photos:FC Photographs 2016:2016 12:2016-12-11:20161211-20161211-_DSC2104-2-Edit.psd” could not be used because the original file could not be found. Would you like to locate it? Previous location: /Applications/Adobe Photoshop CC 2017/LaCie:Photos:FC Photographs 2016:2016 12:2016-12-11:20161211-20161211-_DSC2104-2-Edit.psd
I don't understand the Previous Location in Application.
Now beyond my knowledge, sorry.
Note that the colon symbol ':' is an illegal character for Lightroom so may be upsetting the system if used in filenames.
(I am not a Mac person also)
An what do you mean by "...imported the old one..." ?
Have you tried with both a new catalog AND the original image file on the same internal hard drive (eg. the desktop) of one computer, then edit in Ps?