COVID help- I am trying to get a mold (Cavity & Core) of this face Mask STL file. Can some one help me with this? Or please guide me for making mold using any freeware available?

Here's a step file of the mask. Nobody is going to make a mold of an STL file (unless it is a 3D printed mold).

Trinityscsp is right, the best option here is remaking the model.
I did so by first modifying the STL file. As the model is symmetric, I cut it in half to reduce the facet count. That wasn't enough, so I then decimated the model to reduce facet count even further. More detail can be found in this tutorial: Convert STL (or OBJ) Mesh to SOLIDWORKS Model (NURBS)

The next step was modeling the part in SOLIDWORKS. I won't bore you with the details, but this was a very quick/dirty model. Few dimensions or constraints were used to create the shape. Mostly a bunch of spline sketches, and a boundary surface.
You'd ideally make a prototype to evaluate the form, fit, function of the device. Then once the design was finished, I'd likely make a better, cleaner CAD model with proper dimensions and relationships. But then you'd be paying thousands of dollars for this and not getting it for free on GrabCAD.

Another image is attached showing an overlay between the original, and new CAD data. It is "pretty close". Given the flexible nature of a face mask I doubt it will be an issue.

Before a mold can be made, you'll need to specify the molding process you plan to utilize. If this is injection molding, you could be waiting weeks before tooling is cut and parts are made. I also doubt that any injection molding shops are going to accept homemade tooling. They will design it themselves, for their equipment.

If this is the source of the STL file, be sure to give credit for their work: Face Mask with Replaceable Filter Element