Android Studio Signed APK ':app:transformClassesAndResourcesWithProguardForRelease'

Try performing a clean build (Build > Clean Project) and buliding again.

If that doesn't work, it's likely Proguard is removing some important classes and/or libraries in your app. Take a look at the warnings generated above the error message in the log. You'll need to add rules to your proguard-rules.pro file to keep the necessary classes. Here is an example of the rules required to keep the Butterknife library.