Great news all !!! Ralph Holmes was absolutely right. I had Smart Switch app installed on my MacBook Pro. I had tried android file transfer app in vain, tried all options, debugging, developer options, restarting so many times, yet ..... nothing. I was about to give up, and was contemplating buying an online software. But I was upset, why should I spend money just to sync and transfer files from/to my mobile to/from my PC.
I thought I will give it one last try by removing Smart Switch from my Mac. Then I reinstalled Android File Transfer app and guess what?
yippeeeee........ it works!!!!!!!!!!!!
Thanks Ralph, was good advice. I can not think how one app could prevent another from working, but there you are.