Error running 'app': Unknown error in Android studio 3.1

Just go to "Run/edit configurations.../" and scroll down to bottom of the window and here you see an option "Before launch..." First, remove whatever already inside the little window and then click on (+) icon and select "Gradle-aware Make" and then type "assembleDebug" and select the first option or that you need. This will solve your problem.

The solution that worked for me was a combination of other answers here.

I had upgraded to the most recent stable version of Android Studio at the time, 3.1. Along with that, Android Studio prompted a dialog asking me if I wanted to upgrade my Gradle from 3.x to 4.4, and I accepted as well.

After that I got the error of this question, and then I did this:

1. Open menu Run -> Edit configurations...
2. In the Before launch: section, select any options you have listed (each option has the Android symbol), and remove them with the - symbol
3. Add the option Gradle-aware Make, with the value assembleDebug
4. Clean and rebuild your whole project

just clean and rebuild your project

Try deleting both your build folders in your project manually and rebuild. This usually happens when you update gradle version, in my experience.