Try trashing the prefs which is often necessary after updating Mac OS. These instructions are for PSE 2019 but you can find the corresponding PSE12 files/folders
Launch Finder and go to your Username >>Library >> Preferences
Send the following files/folders to the trash:
Adobe Photoshop Elements 17 Paths
Adobe Photoshop Elements 17 Settings
The library is hidden in recent versions of Mac OS. To get to your user library quickly, from the top menu in Finder, click “Go” whilst holding down the Option (Alt) key and your user Library will appear for you to choose.
One other thing to try:
Click System Preferences in the dock and open Security & Privacy
Choose the Privacy tab and the choose Accessibility
Click the lock to unlock (you may be prompted for your Apple system login password)
Then click the plus button to add the Editor and/or Organizer app. If already listed ensure it has a checkmark. See image below.
To add the Editor:
Macintosh HD >> Applications >> Adobe Photoshop Elements 2019 >> Support Files
Choose (highlight) Adobe Photoshop Elements Editor.app and click open
To add the Organizer:
Macintosh HD >> Applications
Choose (highlight) Adobe Elements 2019 Organizer.app and click open
Re-lock and try launching Elements again. Even if not added to the control list, this often makes a difference after closing Security & Privacy.
Let us know if it works.
Obviously you need to find the PSE12 apps as the above instructions relate to the latest version.