Please use this guide for troubleshooting: https://github.com/keepassxreboot/keepassxc-browser/wiki/Troubleshooting-guide

EDIT: The location of org.keepassxc.keepassxc_browser.json is right at the beginning of the document. To create the file, you must enable some browser in the Browser Integration settings like the browser migration guide shows.

More help and actual issues can be found at https://github.com/keepassxreboot/keepassxc-browser/issues