You may see this error message when building your iOS source code via Xcode. It has a very simple solution. 

In order to support "rich media" notifications on iOS, Apple requires that a Notification Extension be added to your Xcode project. We have already done this for you, but this Notification Extension appears as an additional "Target" in your Xcode project and must be signed separately. 

The easiest fix is to select the Notification Extension target, and sign it the same way you signed your main app's target. If you selected "Automatically Manage Signing" for your main app, please do the same for the Notification Extension.