When I recently updated my Android Studio from version 23 to 24 and I restarted my android studio, then I was faced with this warning in the application build.gradle file.
The buildType section was highlighted in light yellow color. First, I tried to clean up my project and the issues did not go away.
I little Google search reveals what other developers that have experience this android studio warning tried to solve this problem.
But this is what worked for me in my case. It might not work for every use case but I am search it will work in majority of the case.
Steps to follow
1. Click on the File menu while your project is open in Android Studio
2. In the drop down menu, click on Settings sub-menu
3. Click on Build, Execution, Deployment menu
4. Select Build tools
5. Click on Gradles
6. In the window that open, check mark Use default gradle wrapper option
7. Click Apply and OK button and you are done.
If this did not work for you and a different method solved you problem, I will be glad if you can share it with us.