Error:Execution failed for task ':app:dexDebug' after dependency is placed correctly

Question

My studio keeps giving this error:

com.android.ide.common.process.ProcessException: org.gradle.process.internal.ExecException: Process 'command '/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/bin/java'' finished with non-zero exit value 2

however I have gone through questions and i have arranged my grade file as I need to, but the error would not leave. This is my build.gradle:

apply plugin: 'com.android.application'

android {
compileSdkVersion 21
buildToolsVersion "21.1.1"

defaultConfig {
    applicationId "com.apppartner.androidprogrammertest"
    minSdkVersion 14
    targetSdkVersion 21
    versionCode 1
    versionName "1.0"
    multiDexEnabled true
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

dependencies {
compile fileTree(include: ['*.jar'], dir: 'libs')
compile 'com.android.support:appcompat-v7:21.0.2'
compile 'com.loopj.android:android-async-http:1.4.9'
}

what else would i need to do???