In GeneXus, when the Ads (Advertising) are enabled and the selected provider is AdMob, you must fill Ad Unit Id property of the Ads View control. This article explains how to get the identifier for this property step to step.
Register in the AdMob website site to be a publisher.
Go to Home tab, and click on "MONETIZE NEW APP" button.
This will redirect you to another web panel to instantiate a new app.

Once in the new web panel, select "Add your app manually" tab and you must do four steps.
3.a) |
Enter an App name, select the Platform Android.
Then, click on "ADD APP" button and AdMob will give you an App ID (it is not the same as the Ad unit ID).
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3.b) |
After that, select your preferences for the ad format and name ad unit on its respective field.
This will give you an Ad unit ID.
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3.c) |
This step is optional.
In this case, simply click on "SKIP" button.
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3.d) |
Finally, AdMob site allows you to download some tools (there are not necessary) and shows you a summarize of the previous steps.
Click on "DONE" button and finish.
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Once you've done the previous steps, on the left side you can see all your apps with ads.
Select the app that you configure before and it shows you all the ads associated with it.
Simply copying the Ad unit ID number on GeneXus's AdMob Unit Id property, automatically your Android app will have ads on it.

Due creating ads must be validated by Google, AdMob offers Sample Ads Units for each platform which can be used for testing purposes instantaneously.