Please run the app first in real device and if it working then do as follow.
If u have enabled the App Signing Feature in the Developer Console, then it's clearly written in the documentation that the Upload Certificate is only needed by Google for authentication and that upload certificate is removed before installing the application on the user's phone. So, u also need to give the App Signing Certificate SHA under the key which you have enabled for Google Maps API. The App Signing Page is as I have attached the screenshot for your convenience... In the image I have marked which you need to give as additional credentials