Why is my interstitial admob ad not showing up?

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:bust_in_silhouette: Asked By App_develooer

This is the link to the youtube video from where I had learned how to add ads. I had done the same steps as he had done in his video.The banner ad is showing up but the interstitial and rewarded video ad are not. The version of my godot is 3.1.1. Please Help.