First of all, the .json attribute is a property that delegates to the request.get_json() method, which documents why you see None here.

You need to set the request content type to application/json for the .json property and .get_json() method (with no arguments) to work as either will produce None otherwise. See the Flask Request documentation:

This will contain the parsed JSON data if the mimetype indicates JSON (application/json, see is_json()), otherwise it will be None.

You can tell request.get_json() to skip the content type requirement by passing it the force=True keyword argument.

Note that if an exception is raised at this point (possibly resulting in a 400 Bad Request response), your JSON data is invalid. It is in some way malformed; you may want to check it with a JSON validator.