Open your terminal and type:

/Applications/MySQLWorkbench.app/Contents/MacOS/MySQLWorkbench --verbose

if your terminal said:

Your PYTHONPATH points to a site-packages dir for Python 3.x but you are running Python 2.x!
PYTHONPATH is currently: ":/Applications/MySQLWorkbench.app/Contents/Resources/libraries"
You should `unset PYTHONPATH` to fix this.

Type this:

export PYTHONPATH=/usr/local/lib/python2.7/site-packages/    

I solved downloading an older version of Workbench (8.0.20)

One solution worked with me:

System Preferences -> Security & Privacy -> click “Open Anyway”.

And don't forget to unlock the Lock icon at the bottom left.

There are a couple of things potentially going on.

One is the Mac Security and Privacy control, addressed nicely by Mahmoud Mostafa in another answer.

But even once that's gotten around, there is the problem of where this app is looking for Python. Running Catalina 10.15.7, I used the terminal command suggested by Phil Nguyen and got a different error message:

Fatal Python error: initfsencoding: unable to load the file system codec, sys.path = ['/Applications/MySQLWorkbench.app/Contents/Resources/libraries', '/Library/Frameworks/Python.framework/Versions/3.7/lib/python37.zip', '/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7', '/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/lib-dynload'] ModuleNotFoundError: No module named 'encodings'

To my surprise, I saw that there was no Python.framework within /Library/Frameworks/ at all. I'd been using Anaconda, which installed Python under my user directory.

As I had also installed Xcode tools, I just made a symbolic link* within /Library/Frameworks/ pointing to the Python framework at

/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework

and changed the name of the symbolic link to Python.framework. That provides a Python 3.7 instead of forcing things back to Python 2 or using an old version of Workbench. Worked for me; YMMV.

*This needs to be a symbolic link made by ln -s in a Terminal window and not a Finder-generated alias. Working in /Library/Frameworks/ will require sudo.

MySQLWorkbench need Python3.7 Framework in /Library/Frameworks/Python.framework/Versions

1. brew install python@3.7
2. sudo cp -r /usr/local/Cellar/python@3.7/3.7.12/Frameworks/Python.framework/Versions/3.7 /Library/Frameworks/Python.framework/Versions
3. open MySQLWorkbench, it's work

I had the same issue as above. Downloading the older version from archives resolved the issue.

If you are using macOS 10.15 or lesser version just switch to older version of workbench(8.0.11), which is the stabile version and compatible with macOS 10.15 or lesser version. For download Click here

Try running the following command in your terminal, to remove the app from quarantine:

xattr -d com.apple.quarantine /path/to/MySQLWorkbench.app

Note that the default path is /Applications, i.e., the default filename is /Applications/MySQLWorkbench.app