Error CS0117 Clipper does not contain definition for methodes

I'm currently trying to take over an advanced project coded in C#.I'm definitely a novice concerning Visual Studio. The project was left few years ago and nobody holds details concerning the development. The only thing I know is that it was developed with the help of Visual Studio 2015 community edition. I cloned the source code with SourceTree and am now trying to generate an MSI so I can continue developing on a stable and usable version of the code.

After building the solution, I had to install few missing library including Clipper for some of the projects (version 6.4.0 by Angus Johnson). After that, most of the errors concerning Clipper disappeared but, for some reason, I still get unrecognized methods when using them with Clipper objects. I looked up and those methods should definitely exist in the library.

public static Polygons ProcessEvenOdd(this Polygons polygons){    var ret = new Polygons();    var clipper = new Clipper();    clipper.AddPaths(polygons, PolyType.ptSubject, true);    clipper.Execute(ClipType.ctUnion, ret);    return ret;}

For example, here I get:

CS1061 'Clipper' does not contain a definition for 'AddPaths' and no extension method 'AddPaths' accepting a first argument of type 'Clipper' could be found (are you missing a using directive or an assembly reference?)

for (int i = 0; i < insetCount; i++){    // Increment by half the offset amount    currentOffset += offsetBy;    Polygons currentInset = part.IslandOutline.Offset(-currentOffset);    // make sure our polygon data is reasonable    currentInset = Clipper.CleanPolygons(currentInset, minimumDistanceToCreateNewPosition);    // check that we have actual paths    if (currentInset.Count > 0)    {        part.InsetToolPaths.Add(currentInset);        // Increment by the second half        currentOffset += offsetBy;    }    else    {        // we are done making insets as we have no area left        break;    }    if (i == 0)    {        // Reset offset amount to half the standard extrusion width        offsetBy = extrusionWidth_um / 2;    }}

Here, I get:

CS0117 'Clipper' does not contain a definition for 'CleanPolygons'

Is there any reason those could stay unrecognizable for Visual Studio?