Is this happening with any WiFi you try to connect to, or just this one? And is it a public or private WiFi?
One reason could be the number of devices connected to the WiFi system. They aren't unlimited. Especially if it is using an older modem and/or router.
If it is a private modem/router, you could try restarting the modem/router and see if this resolves your issue. You may also want to try disconnecting that WiFi account on the phone, turning off your phones WiFi. Restart the phone. Restart the WiFi and re-enter the information to the WiFi you are trying to access.