Need Help

Dear All,

I am a new learner. I monetize an app with startapp. I already copied the sdk file into /libs, when i tried to generate signed apk, error occured. then I put 

repositories {
    jcenter()
}

dependencies {
    compile 'com.startapp:inapp-sdk:3.8.4'
}

in build.gradle  then error occured with error message: Could not find method compile() for arguments [com.startapp:inapp-sdk:3.8.4] on object of type org.gradle.api.internal.artifacts.dsl.dependencies.DefaultDependencyHandler.
Open File

pleaseee help me how to fix it