您忘记了应用OneSignal gradle插件.根据文档，您应该添加

You forgot to apply the OneSignal gradle plugin. According to the documentation, you should add

buildscript {
    repositories {
        maven { url 'https://plugins.gradle.org/m2/'}
    }
    dependencies {
        classpath 'gradle.plugin.com.onesignal:onesignal-gradle-plugin: [0.10.1, 0.99.99]'
    }
}
apply plugin: 'com.onesignal.androidsdk.onesignal-gradle-plugin'

位于app/build.gradle

应用于文件时，它应如下所示:

When applied to your file, it should look like this:

buildscript {
    repositories {
        maven { url 'https://plugins.gradle.org/m2/'}
    }
    dependencies {
        classpath 'gradle.plugin.com.onesignal:onesignal-gradle-plugin: [0.10.1, 0.99.99]'
    }
}
apply plugin: 'com.onesignal.androidsdk.onesignal-gradle-plugin'


apply plugin: 'com.android.application'

android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "com.myapp"
        minSdkVersion 15
        targetSdkVersion 27
        versionCode 3
        versionName "1.2"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        manifestPlaceholders = [onesignal_app_id: 'XXXX-310c-4776-ae83-XXXX',
                // Project number pulled from dashboard, local value is ignored.
                onesignal_google_project_number: '22978XXXX'
        ]
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.1.1'
    implementation 'com.android.support:cardview-v7:27.1.1'
    implementation 'com.android.support.constraint:constraint-layout:1.1.0'
    implementation 'com.anjlab.android.iab.v3:library:1.0.44'
    testImplementation 'junit:junit:4.12'
    implementation 'com.github.delight-im:Android-AdvancedWebView:v3.0.0'
    implementation 'com.google.firebase:firebase-ads:15.0.1'
    implementation 'com.onesignal:OneSignal:[3.9.1, 3.99.99]'
}