While the AdMob UI asks you to link your app in the
Ad Review Center, it is not required for your app to be in the Google Play Store or Apple App Store in order to place ads from AdMob, so you should be fine. However, when you do publish your app in the Play Store/App Store, it is highly recommended for you to link your app in the Ad Review Center to avoid any issues.
If you have any questions or would like to request for assistance regarding on how to implement the SDK, feel free to let us know and we will be glad to help you.