Admob ads not loading - Failed to load ad: 0

In my case missing payment information was an issue. When you login to Admob dashboard you will see notice over there which says ads won't work until we add payment details.

There is no fault in your code. It's all good. You just need to wait a little. Your ad id is newly created so it will take some time to fetch ads from google servers. You can verify this by adding banner/interstitial ad id you creating for earlier applications and you'll see that they work. So give it some time and it will work soon. At least for me, it happens all the time. Can't surely say for you but why not give it a try?

Constant Value: 0

This error generally occurs in newly created ads. So wait for few hours for the ads to be loaded.