photoshop cc wont open

For Adobe Photoshop CC 2015 under macOS, you want to stay between macOS 10.9.5 Mavericks and 10.13 High Sierra.

While this suppport document indicates that macOS 10.9 Mavericks is required and recommends updating to macOS 10.11 El Capitan, it really should indicate that macOS 10.13 High Sierra is as high as we can go for Adobe Photoshop CC 2015.
https://helpx.adobe.com/creative-cloud/kb/creative-cloud-2015-os-support.html

As you've already noticed with the uninstaller, it's also important to remove old Adobe software prior to upgrading to macOS 10.15 Catalina as neither the installer nor the uninstaller will run once at that macOS or higher.

EDIT:  My memory is fuzzy.  macOS 10.13 is when Apple started to deprecate QuickTime and anything in Photoshop that uses QuickTime may not funcction as expected any longer.  macOS 11 Big Sur is when Photoshop CC 2015 will no longer run at all.

Perhaps try resetting Photoshop preferences?

(read this entire post before acting please)

Resetting restores Photoshop's internal preferences, which are saved when Photoshop closes.

If they become corrupt then various issues can occur.

Here’s some info on how to do that:

https://helpx.adobe.com/photoshop/using/preferences.html

Manually removing preferences files is the most complete method for restoring Photoshop to its default state: 

https://helpx.adobe.com/photoshop/using/preferences.html#Manually

Note re macOS: The user Library folder is hidden by default.

To access files in the hidden user Library folder, see here for how to access hidden user library files.

https://helpx.adobe.com/x-productkb/global/access-hidden-user-library-files.html

Unexpected behavior may indicate damaged preferences. Restoring preferences to their default settings is a good idea when trying to troubleshoot unexpected behaviors in Photoshop. check out the video

https://helpx.adobe.com/photoshop/using/preferences.html#reset_preferences

Learn how to access and modify Photoshop preferences and customize per your frequent workflows

https://helpx.adobe.com/photoshop/using/preferences.html

And here’s an earlier forum discussion as an aid to understanding

https://community.adobe.com/t5/photoshop-ecosystem-discussions/quick-tips-how-to-reset-photoshop-preferences/td-p/12502668

You may want to backup your settings and custom presets, brushes & actions before restoring Photoshop's preferences.

Here is general info about that:  https://helpx.adobe.com/photoshop/using/preferences.html#BackupPhotoshoppreferences

Before you reset your preferences

in case of future issues, I suggest you make a copy as Adobe may need one to check problematic references. 

Quit Photoshop.
Go to Photoshop's Preferences folder

Preferences file locations: https://helpx.adobe.com/photoshop/kb/preference-file-names-locations-photoshop.html\

  [on MacOS see: Users/[user name]/Library/Preferences/Adobe Photoshop [version] Settings

  Note for those on macOS: - be aware that the user Library folder is hidden by default on macOS.

  https://helpx.adobe.com/x-productkb/global/access-hidden-user-library-files.html

  In the Finder, open the “Go” menu whilst holding down the Option (Alt) key.

  Library will now appear in the list - below the current user's “home” directory. ]

Now you can drag the entire Adobe Photoshop [Version] Settings folder to the desktop or somewhere safe as a back-up of your settings.

Note for those on macOS:

Preference preservation is affected by macOS permissions,

you’ll need to allow Photoshop ‘Full Disk Access’ in your Mac OS Preferences/Security and Privacy

I hope this helps

neil barstow, colourmanagement net :: adobe forum volunteer

google me "neil barstow colourmanagement" for lots of free articles on colour management

I'm not sure if this can work on a mac but what happens often on windows is trying to open Photoshop, it shows in Task Manager, but doesn't open. And what makes it work is keeping Task Manager open, force quiting Photoshop, and with Task Manager open relaunching Photoshop.

Have you rebooted and tried?

If you log into another account (may need to make one) does it launch?

Maybe reset preferences:

(macOS only) Open the Preferences folder in the Library folder*, and drag the Adobe Photoshop CS Settings folder to the Trash.

This action only affects the items found in the preferences dialog box. Numerous program settings are stored in the Adobe Photoshop Preferences file, including general display options, file-saving options, performance options, cursor options, transparency options, type options, and options for plug‑ins and scratch disks. Brushes (and lots of other setting) are not affected by the above instructions for deleting preferences. You may wish to make a screen capture of the settings in the Preferences dialog to reset them prior to deleting of this file. 

Also: https://helpx.adobe.com/photoshop/using/preferences.html

*To manually delete Photoshop preferences:

Quit Photoshop.

Navigate to Photoshop's Preferences folder:

MacOS: Users/[user name]/Library/Preferences/Adobe Photoshop [version] Settings

Note:

The user Library folder is hidden by default on macOS. To access files in the hidden user Library folder, see How to access hidden user library files.

https://helpx.adobe.com/x-productkb/global/access-hidden-user-library-files.html

Drag the entire Adobe Photoshop [Version] Settings folder to the desktop or somewhere safe for a back-up of your settings