.stl file is "not ready for printing"
"Not ready for printing" isn't a very specific description of a problem. There are countless things that could be wrong with an STL file that could render it unprintable.

An STL file is basically a collection of triangles in 3D. These triangles need to be manifold/watertight: they need to enclose a volume that represents the object to be printed. For instance, a very simple STL file could contain 4 triangles defining a tetrahedron/pyramid. However, if there is something wrong with the file - for instance, the edges of two triangles might not meet up exactly, then the file is not printable because the triangles, together, no longer enclose a volume.

There are various free and paid tools out there that automatically attempt to fix your STL file. Depending on how bad your file is, you might be able to use one of these tools to fix it. If you do this, be sure to carefully inspect the result to make sure the tool did what you intended. Examples include MeshLab, Netfabb and various online services (Microsoft 3D tools, MakePrintable).

If the problems with your file are so bad they can not be fixed automatically, then the only option will be a - potentially - very laborious manual process of fixing the file. This might mean rebuilding the model from scratch. It is not unreasonable that the company would charge you for this.

Even if your model is manifold/watertight, there are other reasons it might not be printable. For instance, the walls might be too thin, or there might be certain features in the model that can not be printed.

You did not specify the source of your model. Certain pieces of CAD software are more likely to produce unprintable STL files than others, SketchUp is a particularly bad offender. Also, 3D models from games are almost always unprintable. They're designed to look good on screen, but are not designed at all with printability in mind.