I use Visual Studio 2015 Community to program the Astronomy Calculator.  I keep the files on Dropbox for backup but also to have the files available on both my desktop and laptop.  I haven’t done some programming on laptop in a while and when I tried, I got the error message above.  After doing some research, it sounds like issue was uninstalling old versions of Visual on the laptop and leaving some legacy configuration of .NET without the needed files.  The fix is mentioned in this article.