So this error appears quite frequently if you're developing android applications on linux and you're using a physical device for testing purposes. Basically, it means that the Android Debug Bridge doesn't have permissions to access your device.
And here is how you fix that...
First try doing the following:
adb kill-server adb start-server
If this doesn't fix the problem, navigate to the android SDK directory and run the following:
cd /platform-tools sudo chown root:<yourUserGroup> adb sudo chmod 4550 adb ./adb kill-server ./adb start-server
If you've enable the USB debugging on your physical device, a popup is going to show up asking if you want to accept the USB connection, tap on OK and you're done!
Now the device should be good to go.