EDIT Step 1) Go to File > Project Structure > select project > go to “signing” and select your default or any keystore you want and fill all the details. In case you are not able to fill the details, hit the green ’+’ button. I’ve highlighted in the screenshot.
Step 2) VERY IMPORTANT: Goto Build Types> select your build type and select your “Signing Config”. In my case, I’ve to select “config”. Check the highlighted region.