You must have a keystore file to package Apk files. This file is the carrier of your Apk file developer information.

Select the project to be packaged as an Apk file, right click and you will see the export options.

Click Select Export, and you will see a dialog box, where you can choose to export the Android application.

Double-clicking the export Android application will let you select the project and click Next directly.

At this point, you will be asked to select your keystore in the dialog box. There are two options: one is to use the existing keystore, and the other is to create a new keystore.