I'm assuming you're using macOS Catalina? If so, the "iTunes File Sharing" feature is now part of the Finder. When you connect your iPhone, open a Finder window and select it under Locations on the left hand panel and then select the Files tab to access app directories.
Note that the desktop version of the app has some compatibility issues with Catalina related to file chooser dialogs, so the app may not be fully functioning.
The current version 1.x will ultimately be phased out and replaced by version 2.0 in the future and so it's been decided that it's not worth spending the time to go back and fix what macOS Catalina has broken.