Had the exact problem listed below, and was able to resolve it with an answer from someone on the Unreal Engine forum. Others may have listed this solution previously but this post has nice step by step instructions. Worked for me 
Error msg:
UATHelper: Packaging (Windows (64-bit)): ERROR: Could not find NetFxSDK install dir; this will prevent SwarmInterface from installing. Install a version of .NET Framework SDK at 4.6.0 or higher.
PackagingResults: Error: Could not find NetFxSDK install dir; this will prevent SwarmInterface from installing. Install a version of .NET Framework SDK at 4.6.0 or higher.
Solution:
Hello! As mentioned in the output, you need to install/update .NET Framework. You can do this by running Microsoft Visual Studio Installer, then click on “Modify” next to installed version of Visual Studio, switch tab to Individual Components and check latest version of .NET SDK.