Select the text in bottom section of the console to get the complete log (The text near stderr gives proper details about the issue)
First error CommandInvokationFailure: Gradle build failed. C:\Program Files\Unity\Hub\Editor\2018.3.4f1\Editor\Data\PlaybackEngines\AndroidPlayer/Tools\OpenJDK\Windows\bin\java.exe -classpath "C:\Program Files\Unity\Hub\Editor\2018.3.4f1\Editor\Data\PlaybackEngines\AndroidPlayer\Tools\gradle\lib\gradle-launcher-4.6.jar" org.gradle.launcher.GradleMain "-Dorg.gradle.jvmargs=-Xmx4096m" "assembleRelease" stderr[ D8: Program type already present: com.unity3d.ads.BuildConfig FAILURE: Build failed with an exception. * What went wrong: Execution failed for task ':transformDexArchiveWithExternalLibsDexMergerForRelease'. > com.android.builder.dexing.DexArchiveMergerException: Error while merging dex archives: D:\projekty\Fortnite Wyzwania po Polsku\Temp\gradleOut\build\intermediates\transforms\dexBuilder\release\0.jar, D:\projekty\Fortnite Wyzwania po Polsku\Temp\gradleOut\build\intermediates\transforms\dexBuilder\release\1.jar, D:\projekty\Fortnite Wyzwania po Polsku\Temp\gradleOut\build\intermediates\transforms\dexBuilder\release\2.jar, D:\projekty\Fortnite Wyzwania po Polsku\Temp\gradleOut\build\intermediates\transforms\dexBuilder\release\3.jar Learn how to resolve the issue at https://developer.android.com/studio/build/dependencies#duplicate_classes. Program type already present: com.unity3d.ads.BuildConfig * Try: Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output. Run with --scan to get full insights. * Get more help at https://help.gradle.org BUILD FAILED in 8s ] stdout[ :checkReleaseClasspath reBuild UP-TO-DATE reReleaseBuild :compileReleaseAidl NO-SOURCE :compileReleaseRenderscript :checkReleaseManifest :generateReleaseBuildConfig repareLintJar :mainApkListPersistenceRelease :generateReleaseResValues :generateReleaseResources :mergeReleaseResources :createReleaseCompatibleScreenManifests rocessReleaseManifest :splitsDiscoveryTaskRelease rocessReleaseResources :generateReleaseSources :javaPreCompileRelease :compileReleaseJavaWithJavac :compileReleaseNdk NO-SOURCE :compileReleaseSources :lintVitalRelease :mergeReleaseShaders :compileReleaseShaders :generateReleaseAssets :mergeReleaseAssets :transformClassesWithDexBuilderForRelease :transformDexArchiveWithExternalLibsDexMergerForRelease FAILED 21 actionable tasks: 21 executed ] exit code: 1 UnityEditor.Android.Command.WaitForProgramToRun (UnityEditor.Utils.Program p, UnityEditor.Android.Command+WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.Command.Run (System.Diagnostics.ProcessStartInfo psi, UnityEditor.Android.Command+WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.Command.Run (System.String command, System.String args, System.String workingdir, UnityEditor.Android.Command+WaitingForProcessToExit waitingForProcessToExit, System.String errorMsg) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.AndroidJavaTools.RunJava (System.String args, System.String workingdir, System.Action`1[T] progress, System.String error) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.GradleWrapper.Run (UnityEditor.Android.AndroidJavaTools javaTools, System.String workingdir, System.String task, System.Action`1[T] progress) (at <492d67763d4d482e8616a9848b18ccab>:0) Rethrow as GradleInvokationException: Gradle build failed UnityEditor.Android.GradleWrapper.Run (UnityEditor.Android.AndroidJavaTools javaTools, System.String workingdir, System.String task, System.Action`1[T] progress) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.PostProcessor.Tasks.BuildGradleProject.Execute (UnityEditor.Android.PostProcessor.PostProcessorContext context) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEditor.Android.PostProcessor.PostProcessRunner.RunAllTasks (UnityEditor.Android.PostProcessor.PostProcessorContext context) (at <492d67763d4d482e8616a9848b18ccab>:0) UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr) Second error Build completed with a result of 'Failed' UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr) Last error UnityEditor.BuildPlayerWindow+BuildMethodException: 2 errors at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (UnityEditor.BuildPlayerOptions options) [0x00234] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:190 at UnityEditor.BuildPlayerWindow.CallBuildMethods (System.Boolean askForBuildLocation, UnityEditor.BuildOptions defaultBuildOptions) [0x0007f] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:96 UnityEngine.GUIUtilityrocessEvent(Int32, IntPtr)
It shows multiple BuildConfig files leading to a failure. Check if you have built in ads enabled and also ads aar in your project. Having only either of one should fix it.
In case someone else hits this problem - In the unity package manager I had Advertisements version 3, I needed to 'update' it to unity ads v2.08 to get it to work with AdMob and get rid of this dex error.
.aar is the extension used for android libraries. It will have all resources and dex files packed in the single file.
I had the same problem. If you copy the aar file to the libs folder and make the module as in the unity instructions, it may cause an error because there are two aar files. Corrects after deleting aar file in Libs folder