I keep getting the following error: Error:Execution failed for task ':app:compileDebugAidl'. > aidl is missing

Hello,

Google recently released the Android M preview builds and APIs for developers. This is causing problems for people which I believe is what is causing this error. Your best bet is to download API 21, build tools 21.1.2 and update your build.gradle file to use API 21 for compiling, and targetting(this is perfect fine for learning here, but might need some tweaks for actually releasing your app, but hopefully Google will have things sorted out by then). So your build.gradle might look similar to:

apply plugin: 'com.android.application'

android { 

compileSdkVersion 21
buildToolsVersion "21.1.2"

defaultConfig {
    applicationId "your.application.id"
    minSdkVersion 15
    targetSdkVersion 21
    versionCode 1
    versionName "1.0"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

}

dependencies { 
    compile fileTree(dir: 'libs', include: ['*.jar']) 
    compile 'com.android.support:appcompat-v7:21.1.2' 
}

Please let us know if this fixes the issue or not. If not, we can try some other troubleshooting to try to get everything up and running.

Edit Also remember after changing the build.gradle file you should resync your project(Android Studio should prompt you to do so right above the text editing area) and you'll want to clean and rebuild your project(use the Build menu and go to clean, then again to make project).