Hi Subha,
I am in a different time zone to you, therefore I cannot answer your questions in the middle of the night while I am asleep.
Please be patient and just relax. All of this stuff is complex so it is natural that you will not understand it straight away.
Here is the main thing that you are not understanding each time I am responding.
You need to copy the POSE in the VIEWPORT and paste the mirrored version in the VIEWPORT. However, you also need to make sure there are channels to paste into, otherwise you will get an error. (this is what is happening with your arm there are no channels to paste into)
I would suggest that you watch this course first which explains all the things that you are struggling with.
https://cgcookie.com/course/fundamentals-of-animation-in-blender-2-8x
And then come back to the animation bootcamp.
