I think template file is supposed to work for compiling grasshopper addons for rhino6.
And 2015 visual studio for rhino 5.
Is this issue is connected with template file only?
Do you get the same message when you try to create C# Net framework project (without any rhino stuff)?
You can compile .gha files without template too.
Just referencing rhinocommon and gh libraries and changing .dll file name into .gha while compiling in project build properties.