If you get this error and you target only  android or ios environment please check your project file and delete this code block. 

	<ItemGroup Condition="$(TargetFramework.Contains('-windows'))">
		<!-- Required - WinUI does not yet have buildTransitive for everything -->
		<PackageReference Include="Microsoft.WindowsAppSDK" Version="1.0.0" />
		<PackageReference Include="Microsoft.Graphics.Win2D" Version="1.0.0.30" />
	</ItemGroup>