I ran into a similar error and after following the suggested solution and installing .NET 3.1 I was able to to begin a build. However it would fail and give me the error
“UATHelper: Packaging (Windows): 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”.
This led me to this post which pointed me in the right direction.
If you’re still having issues after installing .NET 3.1 open up the Visual Studio Installer and click “Modify” from there open the “Individual Components” tab at the top and search for “.NET Framework 4.8 SDK” and install it. This immediately worked for me.