Company not giving reliving letter.

If your appointment letter clearly states that notice period can be either served or bought out (pay in lieu), then the company cannot legally force you to serve the full period.
If it only says “serve 3 months notice” without mention of buyout, the company has stronger ground to insist.
Send an official resignation email/letter stating:
You have completed 1.5 months of notice.
You are willing to pay for the remaining period as per contract.
Request them to initiate full & final settlement and issue your relieving letter.
This creates a formal record if later you need to escalate.
Absconding will hurt you since you will not get a relieving/experience letter.
But legally, no employer can force you to work against your will. The most they can do is withhold relieving documents.
Send a legal notice through a lawyer, demanding relieving & experience letter on the ground that you offered to comply with contract terms.
You can approach labour commissioner – If your contract allows buyout but company denies, you can complain. Many companies then settle quickly.

Yes it's clearly mentioned in my offer letter that I can pay on lieu the NP. And I have also sended an email for the same but still they are refusing to give me reliving letter.

You first issue a legal notice demanding your relieving letter and settlement of dues,  failing to respond you can approach appropriate legal forum for relief and remedy.