Newbe Question -- not a valid Visual Studio solution file

[Modern_Rocketry]

I downloaded an evaluation version.

The only step I am using is "Build VS.NET Projects"
I have set the filename to:
C:\test\testproject\testproject.sln
The output is:
The selected file is not a valid Visual Studio solution file.
Process completed with exit code 1

Does anyone have any idea where to look to solve this???