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.