My Canon or Nikon camera cannot connect to my Mac OS Catalina 10.15.2 version.
That's Apple's update which made Canon and Nikon cameras not working. You could update to Beta 10.15.3 OS version. You could also use your mobile phone for capturing images, or a specific Sony camera which is compatible. Here are more details on how to use mobile app with Shutter Stream software:
If mobile or a Sony is not an option, first, please back up your files saved on your Mac. Then, best would be to use Time Machine, to restore to the previous OS version. Restart your Mac and immediately press and hold the following combination of keys on your keyboard: "command" and "R" keys. Then, you can restore from Time Machine backup or reinstall a new copy of MacOS.
To install Beta Catalina OS, steps are:
1. If you're not in the Beta program, this might work to see Beta download, copy the following and paste this into the Terminal app (from Launchpad):
sudo /System/Library/PrivateFrameworks/Seeding.framework/Versions/A/Resources/seedutil enroll DeveloperSeed && /usr/libexec/PlistBuddy -c "clear dict" -c "add :SeedProgram string DeveloperSeed" "$3/Users/Shared/.SeedEnrollment.plist"
2. After 5 minutes, go to System Preferences, from Launchpad, then Software Update.
3. Back up your personal files first.
4. Then, press Install Beta, restart Mac to complete installation.