Did you install the required Pattern Detector package to AEM? When
installed, you could test if this url generates a json
will work fine if you have appropriate permissions assigned to it.
Assessing the Upgrade Complexity with the Pattern Detector The Pattern
detector is released as part of the new pre-upgrade-package for source
AEM versions 6.1 to 6.3 and can be installed using the Package Manager.
See these links to pack...