rupali41, I'm not Erin and I am not a moderator, but you can find the explanation for why you can't upload or download in
this thread (Can't post? Can't send a PM? Can't reply? Can't subscribe? Can't download?).
You won't be able to upload or download until you have met certain conditions, which Erin explains in
this post (Unable to download attachments from a Thread). The bottom line is that you have to have a minimum number of posts, reputation points, and days of membership before you can upload or download.