Employer not accepting resignation

I have joined my current company on 21st March 2022 and currently on probation period. Due to some reason, I decided to resign from my position and send mail to the HR mentioning my last working day. However, the HR is not accepting the resignation and saying that we will not release you until we get your replacement. Now the appointment letter says "While on probation, this appointment may be terminated by either side by giving one month’ notice, or one month salary in lieu of notice period. On confirmation, this appointment may be terminated by either side by giving two months’ notice or two months’ salary in lieu of notice period, based on the discretion of the management." When I told the HR that as per my employment term my notice period is 30 days, HR is saying they have mentioned "based on the discretion of the management.", so management has the right to extend my notice period if required. However I have no dues towards the company and no other terms were mentioned in offer of employment. I already got the joining date of my next employer based on this appointment letter and if my notice period gets extended here, I might lose the upcoming opportunity. What can I do in this situation?