Good day @Krishneel,
Are you trying to connect with ubuntu while still connected with another device to that same wifi? It's supposed to let a single device connect to the network.
Some times you'll have to try a couple of times until it connects correctly, connecting and disconnecting until it works. The ubuntu device (specially if you are powering on/off the brain fast) sometimes won't update the wifi network list correctly, you can update that list manually running:
sudo iwlist scan
You could try powering the erle brain 3 off, updatethe list, power it on and try connecting then.