Hi Pasquale,
I can see that Client Association data retention is set to 14 days. That means that every night we purge out historical session data that's more than 2 weeks old. So if you run a report today for June there's no data to report on.
Also, if you ran the report in June, then I'd expect that report to be gone now since the report retention is set to 60 days. So it seems expected to me that the report you want is not available.
Important fact about the Client Session report: Your client association data retention is set to 14 days. If you run a report right now for, say, the last 30 days, then it will only include 14 days' worth of data. If you run it for the last 360 days, it will only include 14 day's worth of stuff. If it's important to be able to run reports for more than 30 days, then you could probably increase the client association retention setting, even if this is for a high traffic public hotspot.
Dan