Sorry, this problem has been resolved…
The problem was simpler: instead of “dragging and dropping” the imported .prehub data to add it to the hierarchy or scene, I was “double-clicking” to expand it.
This is a problem that is more easily overlooked by those who are used to uploading avatars and utilizing Unity on a regular basis, and because it is so elementary, there seems to be very little information on the Internet that explains the procedure without omission.
It was a novice friend of mine who faced the same problem who discovered how to deal with it.
Thank you very much for your sincere help in solving my problem. Please, I sincerely hope that it will reach others who run into the same problem with their first avatar upload.