20 rupees edge error ?

I see a slightly misaligned die.

 

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Since one side of the 12-sided polygon near "भारत" is actually longer than the corresponding sides on a normal ₹20 coin. That suggests the edge itself changed shape, not just the position of the design, hence it is not a misaligned die

 

After doing some research I found out this coin has "Railroad Rim error" a kind of partial collar error

This happens when the planchet (the blank coin) is not fully seated inside the collar (the ring that holds the coin and creates the edge) during the strike. The part of the metal that is above the collar expands outward more than the part inside the collar, creating a "stepped" or "railroad car wheel" look on the edge.
It also has a slight off center core error which is visible in this image

 

That error would be seen on the edge. Since you provided an image of the edge before I look at it and I'm not seeing a partial collar strike of any kind.

 

You are right, it is not partial collar error ,but is a broadstrike error.
Notice how the 12-sided "corners" near the bottom of the coin look less sharp and more rounded/stretched? That is a classic sign of a broadstrike, where the metal expanded freely because the collar wasn't there to stop it.

 

This also proves the error is broadstrike error coin.

 

Somebody please help me identifying the error in this coin.

 

We have all tried. If nobody else responds then they don't think it's a mint error of any kind

I'm sorry to say this but give it up already.
In my honest opinion it's not a mint error of any kind.

I don't think it's Broadstruck at all

 

I have finally found it.

The error is Uncentered Broadstrike.
This coin was struck without the full confinement of the 12-sided retaining collar. While a standard ₹20 coin is constrained to a perfect 27 mm dodecagon, this specimen was struck "uncentered," causing the metal to flow freely toward the side with the "भारत" (Bharat) lettering.

2. Evidence of the "Stretched" Edge (The Bharat Side)
The most definitive proof is the geometric distortion of the dodecagonal rim(this is visible through the parallel lines passing near भारत ) . If you measure the 12 sides of the polygon, the sides adjacent to the "भारत" lettering are physically longer than the other sides. Because the collar was not present to stop the metal flow on this side, the pressure of the strike forced the metal outward. This created an elongated edge that causes the coin to overlap a standard ₹20 coin when placed together.

3. The "Normal Diameter" Paradox
The coin retains a normal diameter on the side opposite the error. This is characteristic of an Uncentered Broadstrike, where the collar was still partially engaged or very close on one side, but completely bypassed on the other. This results in an asymmetrical, slightly oval expansion rather than a uniform increase in size.

 

Good for you. I still don't think it's an error though.

With you be sending it to a third party grader for Attribution?

 

No I don't have money for that, I actually sell my coins to coin collectors at a regular Sunday flea market in the city where I live.