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.