Unable to build apk: The number of method references cannot exceed 64K

build.gradle 파일 내 defaultConfig 에 multiDexEnabled true 를 입력하면 해결

android {
    compileSdkVersion 23
    buildToolsVersion "23.0.3"
 
    defaultConfig {
        applicationId "com.gkwak.***"
        minSdkVersion 15
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
        multiDexEnabled true // 이곳에 추가
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

참고 : https://developer.android.com/studio/build/multidex.html