admob getting an ad response. ErrorCode: 0 Failed to load ad:0

Question

I have followed many answers about that error here, but my problem is not solved. Every time I run my app no ads appear and I get this error message in Logcat:

Answer 1

I added the code below for the testing environment

 AdRequest adRequest = new AdRequest.Builder().addTestDevice(your device id)
 //When you testing on emulator
.addTestDevice(AdRequest.DEVICE_ID_EMULATOR).build();