I can't build to android! It says Gradle Build Failed

For some reason I wasn’t able to build to android. I have everything set up, it worked on other projects, but not this one. I’m not sure what caused it Here’s the error it generated.

CommandInvokationFailure: Gradle build failed. 
/Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home/bin/java -classpath "/Applications/Unity/PlaybackEngines/AndroidPlayer/Tools/gradle/lib/gradle-launcher-4.0.1.jar" org.gradle.launcher.GradleMain "-Dorg.gradle.jvmargs=-Xmx2048m" "assembleRelease"


FAILURE: Build failed with an exception.

* What went wrong:
A problem occurred configuring root project 'gradleOut'.
> Cannot add task ':prepareUnityAdsLibrary' as a task with that name already exists.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.

BUILD FAILED in 1m 22s
Starting a Gradle Daemon (subsequent builds will be faster)
NDK is missing a "platforms" directory.
If you are using NDK, verify the ndk.dir is set to a valid NDK directory.  It is currently set to /Users/rizki/Library/Android/sdk/ndk-bundle.
If you are not using NDK, unset the NDK variable from ANDROID_NDK_HOME or local.properties to remove this warning.

exit code: 1
UnityEditor.Android.Command.WaitForProgramToRun (UnityEditor.Utils.Program p, UnityEditor.Android.WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg)
UnityEditor.Android.Command.Run (System.Diagnostics.ProcessStartInfo psi, UnityEditor.Android.WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg)
UnityEditor.Android.AndroidJavaTools.RunJava (System.String args, System.String workingdir, System.Action`1 progress, System.String error)
UnityEditor.Android.GradleWrapper.Run (System.String workingdir, System.String task, System.Action`1 progress)
Rethrow as GradleInvokationException: Gradle build failed
UnityEditor.Android.GradleWrapper.Run (System.String workingdir, System.String task, System.Action`1 progress)
UnityEditor.Android.PostProcessor.Tasks.BuildGradleProject.Execute (UnityEditor.Android.PostProcessor.PostProcessorContext context)
UnityEditor.Android.PostProcessor.PostProcessRunner.RunAllTasks (UnityEditor.Android.PostProcessor.PostProcessorContext context)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

I’m not sure why the build type is Gradle instead of Internal so I changed it back to Internal. But it still gave me this error CommandInvokationFailure: Unable to convert classes into dex format.

Did you download the android SDK and the android packet when you installed unity?