You got this error because rippleColor is defined in MaterialDesignLibrary and there are dependences in your project that already defined the same attribute. Please check it in your build.gradle. It could be com.android.support:design or com.google.android.gms:play-services.
The solution is to only include the library you need, instead of adding the whole library.
In my project I use com.android.support:design + com.android.support:appcompat instead of the material library.
Also see Gradle error: "Attribute "rippleColor" has already been defined" in android studio