Using Photoshop Elements Editor? Mac App Store version? You will probably need to re-install. Your download should still be available in the Mac App Store under your "Purchases"
Apple has had problems with expired security certificates which have affected all apps downloaded from the Mac App Store. We are told by Apple that all certificates have now been renewed well into the future; so this error should not happen again. However some people are still experiencing problems. Here is one solution, reported on the Adobe Community Forums, which has worked for several users. Give it a try by following each step:
1. Send the Elements app to the trash
2. Click the trash can in the dock and empty the trash
3. Go to the App Store and sign-out
4. Re-boot your Mac
5. Return to the App Store and sign-in again with your ID
6. Go to your purchases and download and install the Elements Editor app again.
This fix appears to work for most users although a few are required to clean Group Containers with the exception of the catalog. See this Adobe help link: