The screenshot you shared is looking for:
When Client Count is at most (not exceeding) 40 clients for a 5 min window on a single Radio or Interface with SSID1. This means the single AP radio has to be the AP of association for the 40 clients.
Are you trying to isolate it per AP? Or just across all APs that have the same BSSID?
If it's the latter, then you might try changing the 'limit by' from Radio/Interface to BSSID which would cover that SSID globally across all radios.
There's also a feature request in there to choose which hours this trigger is in effect for. As it doesn't make sense to have this trigger if the clients aren't expected on the network 24 hours a day. I'm just guessing, but most likely looking for client behavior 5 days a week from 8am - 5pm (give or take) - the ability to set a time window for an alert to be active doesnt exist currently.