Resignation without notice period - Company had responded saying they cannot accept my resignation

Hi team, I joined a company 3 months back, I was on probation for a period of 6 months. However, the work culture and the shifts were taking a toll on me and I started falling sick quite often. Due to my health reasons, I had to resign at the earliest. I have mailed and posted my letter of resignation to the Employer with an effective date of resignation and also stated that I want to return the Assets so as to guide accordingly. After 3 weeks, the Company had responded saying they cannot accept my resignation without serving a notice period of 3 months and asked me to return the assets at the earliest and once I return the assets, to label me as an absconder. Advise me.

You have to serve the notice period which you had agreed while accepting the employment. If you serve the notice period then you cannot be labelled as absconder. Give a clear reply indicating your notice period and your date of relieving in accordance with that, or offer to buy out the remaining notice period after the proposed
last day of service.
You have no right to retain the company assets, particularly when it is asked for. There can be legal proceedings against you for defaulting in depositing back company assets.