order_response = {
"orders": [
    {
        "id": '1',
        "email": "b@mail.com",
        "location_id": 9,
        "line_items": [
            {
                "id": 5,
                "product_id": 6,
            }, {
                "id": 7,
                "product_id": 8,
            }
        ]
    }, {
        "id": '2',
        "email": "b@mail.com",
        "location_id": 10,
        "line_items": {
            "id": 3,
            "product_id": 4,
        }
    },
]

}

id email      location_id line_items_id line_items_product_id
1  b@mail.com 9           5             6
1  b@mail.com 9           7             8
1  b@mail.com 10          3             4

pd.io.json.json_normalize(report_json, ['line_items'], ['id', 'email'], record_prefix='line_items_')

report_json = {
"orders": [
    {
        "id": '1',
        "email": "b@mail.com",
        "location_id": 9,
        "line_items": [
            {
                "id": 5,
                "product_id": 6,
            }, {
                "id": 7,
                "product_id": 8,
            }
        ]
    }, {
        "id": '2',
        "email": "b@mail.com",
        "location_id": 10,
        "line_items": [{
            "id": 3,
            "product_id": 4,
        }]
    }
]

}
#print (report_json)

df = json_normalize(report_json['orders'],
                    ['line_items'],
                    ['id', 'email'],
                    record_prefix='line_items_')
print (df)
   line_items_id  line_items_product_id id       email
0              5                      6  1  b@mail.com
1              7                      8  1  b@mail.com
2              3                      4  2  b@mail.com