I recently found myself wanting to convert a Chrome Extension to be used in safari. I couldn’t get the command to run properly, I found I needed to type the following in my terminal to select the correct version of Xcode first:
sudo xcode-select -s /Applications/Xcode.app
You’re then able to run the converter as expected:
xcrun safari-web-extension-converter <PATH_TO_CHROME_EXTENSION_FOLDER>