First, on your comment about notice period waiver, it is completely upto the employer if they want to waive off your notice period or not even if you are ready to pay for the shortfall (Again, please check your offer letter for this as well). However, in most of the cases, on medical ground employer does not deny on this front. But, in any case, you can not force or claim on this part.
Second, if the company has accepted your resignation and you have valid document confirming acceptance of your resignation, and also company has confirmed that your notice period is waived off / or you have served the notice period and your last working date is "__" mentioned anywhere in any of the document and that date has passed, then you have full right to claim for your releiving letter. In this case, the company "legally" can not deny from issuing releiving letter to you.
If you wish, you may proceed legally against this company (again, only if you have valid documents confirming your last working date and you have served till that date).
10th May 2019 From India, Pilibhit