Make sure you have Cordova properly configured with the corresponding android or ios platform (see the official docs).

cordova plugin add cordova-admob

See Cordova CLI (Command-line Interface) docs

phonegap local plugin add cordova-admob

To use in Phonegap Build, place the following tag in your config.xml file:

<gap:plugin name="phonegap-admob" source="npm"/>

ionic plugin add cordova-admob

Note: No ads will be served on apps with package name io.ionic.starter. This is the default package name for new ionic apps. Make sure to rename the package name so ads can be displayed.

See Ionic Framework docs