Unable to export all objects to a STL in AutoCAD
Based on your error messages, it's likely you have created a non-manifold 3D object. As a simple example, let's start with a cube as suggested in your question. To keep things simple, you want to add a cylinder to the cube. 

In other programs, one would create a sketch on the surface of that cube and extrude the circle into a cylinder. Because the circle is constructed on the surface, it become an integrated part of the model.

If you had created the cylinder independently, which can be done in AutoCAD and placed it on the surface of the cube, the resulting model would appear to be the same as the one created above.

The placement method may have resulted in the end of the cylinder "penetrating" the surface of the cube. Such a model may generate the face-face error you've received.

The quantity reference you've provided in your question indicates that the problem is contained in more than one intersection.

If you have patience, you can return to the original model and attempt to locate piercings or penetrations and adjust the locations to be joined properly. Selecting a wireframe view may assist you in those efforts.

Another option would be to import your .STL file into a program such as Meshmixer to attempt an automatic repair. Under Analysis/Inspector, errors will appear with flags which can be clicked to perform a repair.

Severe errors in construction, such as the one you describe, are more likely to destroy the model when using this method, however.