Delay in unconditional offer letter

I have meet all of my conditions. It has been six days I am not getting the unconditional offer letter. Is it a problem or something else? .

6 days, (and I am presuming less than that in working days), is not unsual at all. Your uni will be processing many thousands of documents at this time of year. 1-2 weeks, (or more) is common.
Call the uni if you want to confirm timescales for processing, (don't email, it just exacerbates things.)

Thanks. Can you plz tell me is there any chance to get rejected after getting conditional offer?

No, not unless you don't meet the offer conditions, or they find you lied on the application somewhere.

Last question : How much time it will take to give unconditional offer letter?

Impossible to say as it depends on that uni's backlog of documents to process. Could be a day, could be a month or more.
As above, call the uni if you want to confirm their current timescales.

But my class will Start in 9th September... If it late to much when I will apply for my visa?

There's still plenty of time, most unis have only just started issuing CAS's. Many international students won't be able to meet their conditions until late August.
In your case, in all likelihood your documents will be processed within another week or two.