google admob is not showing. Error "ad failed to load 3"

Hi everybody.
I am a game developer. I have a problem about showing ads on my project.
I have found a lot of the way to show ads from google.
For now, I am using the way from link:

The popup have shown on my project, but the ads is not showing.
From logcat, I get a error notify: “ad failed to load 3”
I have searched a lot of the way from google, stackoverfollows, but all are not successfull.
I want to the support from everybody.
Thank everybody so much.