like below fixes the issue I just downloaded the project. First look at the settings.gradle:
include ':app', '..:DFULibrary:dfu'
compile fileTree(dir: 'libs', include: ['*.jar'])
compile project(':..:DFULibrary:dfu') // <-- You do not have this
Gradle Project sync failed - Android Studio Gradle Build 2.20
Does that help There is a bug for Android Studio. I guess your PC's language is Turkish. If you write "androidTestApi" include "androidTestImplementation" to your gradle file, Android Studio gives WARNING like this:
Gradle Sync Error - Importing Gradle Project into Android Studio from Unity