Unable to resolve host "<URL here>" No address associated with host name

It is WiFi bug due to wifi disable or not properly connected.

Simply Reconnect the wifi will solve the issue.

I've seen this problem in the emulator as well. In my case, it is caused by launching the emulator, then putting the computer to sleep or changing the network connection (going from work to home, etc), then attempting to use the same emulator again. Closing and re-launching the emulator resolves the problem in this case.

You probably don't have the INTERNET permission. Try adding this to your AndroidManifest.xml file, right before </manifest>:

<uses-permission android:name="android.permission.INTERNET" />

^{Note: the above doesn't have to be right before the </manifest> tag, but that is a good / correct place to put it.}

^{Note: if this answer doesn't help in your case, read the other answers!}