Problem with editing - 3D print

Hello! Your models are really nice, but if I wanted to print them on my 3D printer Anycubic, I have the problem with every try to edit every model :( Where is the problem? I wanted to reduce faces in Fusion 360, but i wrote me, invalid input :( 3ds Max too impossible to do something... I am lost :( Please could you help? I love your models so really much and I want to put them on my shelf.

Unfortunately, I do not do 3D printing and can not advise anything.

Perhaps someone from the visitors have experience and they will talk about solving the problem.

I just need transfer .stl file to some CAD formate like .iges, .step. I must tranfer it into solid (NURBS). I also cant print wires on the ships. But If I am trying to convert it, it always write: error, invalid input or something like that.

I just need somehow edit this models. I tryied Blender, Fusion 360, 3ds Max, Autodesk Inventor and it is impossible to tranfer it into solid model (NURBS - iges or step file). Macтep could you write me pls, which program do you used? 

I just convert models from the game files into a special format for the site. Models are converted "as is". That is just as a set of vertices and faces.
I do it by the programs I wrote myself.
Probably, problems are caused by the fact that these models were not originally intended for printing.

So is there some program, where I can edit it? Thank you! :)