What's the question?
Hi, I use TestComlpete 12 dlls in my Visual Studio solution.
If I compile solution as .NET 4.5.2, everything works fine but
if solution compiled as .NET 4.7.1, AutomatedQA.script.var is not supported.
Note that connected applications have been deprecated in favor of the TestLeft solution. They are still supported but you should look at moving to a different methodology.
That said, according to the documentation, coding connected applications with Visual Studio only supports 2013 and earlier. I'm not sure where the .NET cutoff is but this might play into the problem you're having.