Skipping bolts on seatpost , please help?
₹ 555 (Indian Rupees) is about $11 NZD or about $7.50 US, 5.64 UKP or 6.27 Euros.
So you've got a seatpost that is incredibly cheap and likely made of low-grade metals.  Zooming in a bit shows the terrible surface finish, and the inside won't be any better.

By repeatedly alternating between tight and loose, the threads inside have become &quot;stripped&quot; and there's not enough metal left to hold the bolt.    I bet you found little half-moons of metal flaking out of the hole when removing the bolt fully.
There's a chance you over-torqued the bolt too, which uses the bolt's thread as a kind of lever to force the metal threads off the sides of the hole.
You may be able to put a wider bolt in after re-cutting a new thread with a tap, but given the cheese-nature of the metal, it won't last very long.  You can also fit a replacement thread, but the tools for that cost more than a decent seatpost.
On the other hand, if its the bolt which is missing its threads, and the threads inside the clamp are still okay, a replacement bolt would resolve the problem for now.  Do make sure its a quality bolt.
Buying a better-quality seatpost will fix your problem.  For me, a low-price good one is four to five times as much.   You don't need to spend excessively, but based on the pricing and what has happened, it was a piece of junk.   You might also explore returning the item, though that is up to you.

Your second photo clearly shows no threads left in the square &quot;nut&quot;  If you could replace that with a thicker nut it would hold for longer, but some other part could fail too.  I would expect the pressed metal rail holder to give up next, or maybe the bolt itself.