The same issue was posted here but the answer given didn\'t work for me. I\'ve uploaded to the store before and now I can\'t update my app to include some bug fixes. Wheneve
I assume that you have updated your android studio to 0.3.6.
check this one:
Android Studio generate signed apk not working on build 0.3.6 - Giving Error
and also as 6thSigma said: run "gradle assembleRelease"