finder cant open files

Another thing you could try....

Try deleting the Finder preference File.

In Finder, press Command-Shift-G and paste this

~/Library/Preferences/

Delete the file: com.apple.finder.plist

Drag the file to the trash, then force-quit the Finder, or log out and back in.

Force Quit: Click Apple logo in upper left menu or key ‘option + command + esc’

First thing is to restart your mac. Odds are that this is a temporary glitch and restarting will fix it. If the issue persists: try restarting in Safe Mode. It will take longer to boot and will clear system caches and do a few checks. It works slowly. Does the problem still occur in Safe Mode? Does it reappear if you then restart normally?

When I saw Randall’s suggestion I expected the problem to be solved. Now I think that either your drive is failing or there is a third party software wreaking havoc on your system. Please run Etrecheck and post its full report here.

Use the “additional text” button and paste the report into the text box.

What type of files are these? What is their extension?

The Finder doesn't open files, the application that created the file does. Select one of the files and bring up its Info window (⌘+i) and see what app is listed as the app to open it with.