You should look into the ‘json key’ and ‘json value’ inspectors at https://developer.bigfix.com/relevance/reference/json-key.html
There are also a couple of working examples in this thread at Json inspector question
One common hangup I see with JSON parsing is when one expects it to behave like Registry inspectors. Bear in mind that we don’t retrieve key "X" of key "Y" of jsons of files "/tmp/test.json" , instead we retrieve key "X" of value of key "Y" of jsons