When ever I try to run my project I get this error:

C:\Program Files (x86)\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(2,2): Error MSB4018: The "Aapt" task failed unexpectedly.

System.InvalidOperationException: Sequence contains no elements

   at System.Linq.Enumerable.First[TSource](IEnumerable`1 source)

   at Xamarin.Android.Tasks.BuildToolsUtils.GetBuildToolsPath(String androidSdkDirectory)

   at Xamarin.Android.Tasks.Aapt.GenerateFullPathToTool()

   at Microsoft.Build.Utilities.ToolTask.ComputePathToTool()

   at Microsoft.Build.Utilities.ToolTask.Execute()

   at Xamarin.Android.Tasks.Aapt.Execute()

   at Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()

   at Microsoft.Build.BackEnd.TaskBuilder.<ExecuteInstantiatedTask>d__20.MoveNext() (MSB4018) (test)

And I dont know how to fix it.
What is wrong and how do I fix it.

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