You have to use your full Ad Unit ID. An Ad Unit ID looks like this ca-app-pub-1520537222610272/9864485652. You can create an Ad Unit ID in admob.com > your dashboard > monetization > your app.EDIT: Merging your topics.
Could you post your logs (Debug > Logs > Generate Logs)?
In your case, Stencyl is trying to use an old version of the AdMob SDK. To fix this error, do a clean install of Stencyl. To do a clean install you have to uninstall/delete your current version of Stencyl, download the latest private release and install it. Do NOT install the latest version of Stencyl over your existing one! After you have done a clean install of Stencyl, use Run > Clean Project. This should fix your issue.