Have you implemented the Google Mobile Ads SDK properly?
Are you trying to implement mediation?
Have you implemented 3rd party network SDKs and adapters?
Have you set up AdMob mediation in your AdMob interface?
Does the format of the ad code match the format of the ad request? i.e., Are you using your banner ad code in the banner ad request in your implementation? Interstitial ad code for your interstitial ad request?, etc.