buildscript {
    repositories {
        google()
        mavenCentral()
        maven { url '<https://devrepo.kakao.com/nexus/content/groups/public/>' }
    }

allproject{...}이 사라지고 아래처럼 해야한다.

dependencyResolutionManagement {
    repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
    repositories {
        google()
        mavenCentral()
        jcenter() // Warning: this repository is going to shut down soon
        maven { url '<https://devrepo.kakao.com/nexus/content/groups/public/>' }

KAKAO_SDK_GROUP=com.kakao.sdk
KAKAO_SDK_VERSION=1.27.0

-keep class com.kakao.sdk.**.model.* { <fields>; }
-keep class * extends com.google.gson.TypeAdapter

<string name="kakao_app_key">앱 키</string>

implementation "com.kakao.sdk:v2-user-rx:2.8.4" // 카카오 로그인
implementation "com.kakao.sdk:v2-talk-rx:2.8.4" // 친구, 메시지(카카오톡)
//implementation "com.kakao.sdk:v2-story-rx:2.8.4" // 카카오스토리
implementation "com.kakao.sdk:v2-link-rx:2.8.4" // 메시지(카카오링크)
//implementation "com.kakao.sdk:v2-navi:2.8.4" // 카카오내비
implementation group: project.KAKAO_SDK_GROUP, name: 'usermgmt', version: project.KAKAO_SDK_VERSION

<meta-data
      android:name="com.kakao.sdk.AppKey"
      android:value="@string/kakao_app_key" />
  <activity android:name=".MainActivity">

• MainActivity.java