Androidアプリケーションで用いるデータベースとしてRealmを導入する時にハマった場所についての記事です。デフォルトで自動生成されたbuild.gradleのprojectの方を開き、dependenciesの３行目に

classpath "io.realm:realm-gradle-plugin:10.0.1"

を追加します。

build.gradle(project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = "1.4.31"
    repositories {
        google()
        mavenCentral()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.2.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "io.realm:realm-gradle-plugin:10.0.1"
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

ここでSyncすると、以下のようなエラーメッセージが出ます。

A problem occurred configuring root project 'MyApplication'.
> Could not resolve all artifacts for configuration ':classpath'.
   > Could not find io.realm:realm-gradle-plugin:10.0.1.
     Searched in the following locations:
       - https://dl.google.com/dl/android/maven2/io/realm/realm-gradle-plugin/10.0.1/realm-gradle-plugin-10.0.1.pom
       - https://repo.maven.apache.org/maven2/io/realm/realm-gradle-plugin/10.0.1/realm-gradle-plugin-10.0.1.pom
     Required by:
         project :

Possible solution:
 - Declare repository providing the artifact, see the documentation at https://docs.gradle.org/current/userguide/declaring_repositories.html

ここで先程のbuild.gradle内のrepositoriesに jcenter()を追加すればビルドエラーが消えます。

build.gradle(project)

// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {
    ext.kotlin_version = "1.4.31"
    repositories {
        google()
        mavenCentral()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:4.2.0'
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
        classpath "io.realm:realm-gradle-plugin:10.0.1"
        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}

出力

BUILD SUCCESSFUL in 10s