The Javac task failed unexpectedly. System.IO.PathTooLongException

To resolve this issue you need to edit the your Android CS Project by adding this two

<PropertyGroup>
    <UseShortFileNames>True</UseShortFileNames>
</PropertyGroup>
<PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <IntermediateOutputPath>C:\temp</IntermediateOutputPath>
</PropertyGroup>

Then, reload your project and rebuild.