Compilation is not supported for following modules: Unfortunately you can t have non-Gradle Jav...

问题

Compilation is not supported for following modules:
Unfortunately you can t have non-Gradle Java modules and Android-Gradle modules
in one project.

解决方法 ：

1- close the project

2- close Android Studio IDE

3- delete the .idea directory

4- delete all .iml files

5- open Android Studio IDE and import the project

The answer is taken from HERE