According to this answer How to solve "Your APK's version code needs to be higher than 2." in Google Play's Developer Console? I have just changed the vers
Sometimes the PlayStore encounters a bug when you upload an APK during a Release creation, and you are stuck because you cannot upload the same APK again, for the current draft release. You get the error "Upload failed..."
The solution is to go to the Artifact library menu, under Release management and to delete the draft artifact. Once this is done, you will be able to upload the APK again, and to finish your release.
Hope it will help others...
Ben