At for the new MacOS Sierra, you may want to visit this topic: MACOS SIERRA PROBLEM: SLOW WIFI – WIFI NOT WORKING [FIX]
In a particular case that you can not access to a WiFi network even using the right password which has not been changed since you previously connected successfully, follow the below solution to fix:
Go to System Preferences > NetWork > Click on “Advanced…” button > Select the WiFi network in trouble > Click on (-) button to remove it from the Wifi list recorded > then attempt to reconnect again.
Otherwise, Whatever the your WiFi issue is, you probably can fix the problem through a simple command line in Terminal:
- Find Launch Terminal (by pressing Command + Space keyboards then type “Terminal”) and paste the following command then Enter.
sudo ifconfig awdl0 down
- Fill the user password (Mac OS X user password, not Wifi password) > Enter.
This tip might not work on all circumstance because many Mac users complained they had Wi-Fi bug related to a failing AirDrop file during transmission or when the connection is slow. Hopefully Apple will solve this problem in OS X El Capitan when it is released officially.