What is the buildToolsVersion for android SDK 24?

buildToolsVersion

Using android gradle plugin version > 3 you can remove the android.buildToolsVersion property

Quoting from the New Features section of the Android Gradle plugin release notes for version 3.0.0:

You no longer need to specify a version for the build tools. By default, the plugin automatically uses the minimum required build tools version for the version of Android plugin you're using.

[minSdkVersion, targetSdkVersion, compileSdkVersion]

buildToolsVersion should be set to 25 and 25.0.0, as seen below:

compileSdkVersion 25
buildToolsVersion '25.0.0'
useLibrary 'org.apache.http.legacy'

defaultConfig {
    minSdkVersion 15
    targetSdkVersion 25
    versionCode 1
    versionName "1.0"
}

You can get the latest buildToolsVersion from SDK Manager in Android Studio.