Hello,

A keystore file is a file that stores a security key and it is used to sign your Android apps. Signing the app is the way to identify the author of the application for an Android app.

In order to generate the APK file using a Release Build Type, you need to configure a keystore file property in your building configurations.

You can do this in Service Center, if you go into the Distribute / Native Platforms tab:

Please, follow this link that will provide you a detailed guidance in order to create this file:
https://success.outsystems.com/Documentation/11/Delivering_Mobile_Apps/Generate_and_Distribute_Your_Mobile_App/More_Information_on_Generating_and_Distributing_Mobile_Apps#create-a-keystore 

After that, you should be able to upload the file to the Service Center configurations and generate your APK file successfully.

Hope that this helps you.

Kind regards,

Rui Barradas