I have been away from the Creation Kit for a while and when I tried to edit a custom NPC or any NPC, for that matter, neither the Skeleton or the FaceGen show up in the render window. Therefore, I cannot track any changes that I make to the NPC. Why is that?

I think you are looking for the last three tabs in the actor window.

The preview button will update and show the character's face or body depending on what you selected.

Unfortunately it didn't. For some reason neither the full body or the FaceGen show up in the preview window. Have no clue what that is about.

Must be just a bug that can't be fixed. Wonderful!

FYI:

Since this is a freshly reinstalled game I found that once I installed XPSE and all of its required mods I now see a purple shadow FaceGen in the rendering window and only the head part in the full body rendering window. Isn't there someone there that can help me with this issue? I would really appreciate the help. Thank you.

Purple always means missing textures. Make sure all files were installed correctly.