Reports Query option period='D7/D30 etc' was returning results but has been fixed, see this thread. You can try this again but NOT on Graph Explorer.
Graph Explorer does not yet support csv body, so it will try to JSON parse the CSV and hit an error which shows on your screenshot. To test this out, use Postman which will handle and display the CSV response. See screenshot below.
The only issue that is also on this thread is that the values of Display Name and may be Site Url returns some uuids. Please try and check and upvote the question.