I had an interview before 10 days. At that time the employer told me
that " you are confirmed". But still I am waiting for the offer
letter. In the mean while i contacted the employer through phone and
they replied that, " our boss is not in office he will back in near
days". So my question is, Is this genuine reason by the employer or is
it trap ?
I understand your position, When you are waiting on a job offer after a interview every time the phone rings you will looking at the caller id saying-"Is this the call? And then it turns out to be XYZ.
I would suggest as others pointed keep your job search open."Even if you're certain an offer is coming, do not stop job hunting," The longer they make you wait, the lower the chances of appointment.
There's nothing you can do but wait. Calling them (or emailing) repeatedly isn't going to speed things up. If key personnel are out of the office or whatever, calling won't change that. You don't have a start date until it is in writing, and if they continue to delay, they should also be pushing back the start date.
"Internal processes can take weeks – often much longer than the whole initial interview process."
All the best, do let us know here in this thread or chat-room once you get the offer letter.