Hi...
If you are using the Windows version, Photoshop will be installed at the location below.
C:\Program Files\Adobe\Adobe Photoshop CC 2019\Photoshop.exe
So just confirm you have Photoshop installed in the same location or locate it by running a search.
Then open up Preferences (CTRL+P) in CTA3 and then click the "Open File" button in the Texture Editor box.
Now navigate to where Photoshop is installed, select Photoshop.exe and click Open. That should be it.
Peter
Forum Administrator
www.reallusion.com