According to Google Play latest changes, at the moment you have an option to Enroll to Google Play App Signing.
Please do not enroll in this program right now. We still need to investigate all the pros and cons.
If you Enrolled in App Singing Program, you won't be able to upload new APK signed with our KEY STORE FILE.
* We do not provide our KEY STORE FILE
If this is a new app, delete the app from Google Play Store, create new one ( Do not Enroll in Google App Signing ), request a new submission process, upload the APK.
You will be able to upload the APK file successfully without enrolling into the App Signing Program.