As stated before, 3DS Max offers several tools to generate and modify 3D models, with simple points or predefined models (primitive shapes). It is possible to combine those different elements, to obtain models adapted to your needs.
It’s important to know the state of our object in the 3DS Max language. Thus, primitive shape is considered as an object on its own, not modifiable in its state. Then a lot of menus (like those in the modifier “Edit Poly”) are masked.
That’s why it is necessary to convert this object as a Mesh, or a polygon to get access to all the Modifiers that we saw previously. If you use a software such as Rhino 3D, and you’re used to Nurbs, you can convert your object in a Nurb in this same menu.
If they are not available, or if they are masked, it is just that the selected object is not a Mesh or a polygon, and then you can’t modify it.
After the addition of a Modifier or a Bolean operation between two parts of the model, it’s not rare that the object is considered as a primitive form, that you can’t modify. Check permanently the state of the model to avoid every issue during the modification.