Stuck Without a Relieving Letter: How Can I Get HR to Respond and Help Me Move On?

I am working for a company, and I resigned from there. Everybody from the admin tried to compensate to make me withdraw my resignation, but I declined to withdraw my resignation. I started serving my notice period.

One day, I got an email from the HR department stating that we are relieving you today, and after all the no dues clearance, they emailed me to come to the office after 2 days to collect my relieving letter. After that day, HR is not responding to the email. They are also giving excuses like the CEO is not signing the letter or similar reasons. Sometimes, they say that the CEO wants to meet personally, but he has no time and will inform me once he has time.

I need to join another company, and without the relieving letter, I will not be able to join the new company. Please suggest what options are available for me on how to get my relieving letter as HR is not responding to the email sent by them.

I have all the clearance and no dues via email from every department. I have also received an email from HR asking me to come to collect the relieving letter on a specific date. However, HR has not responded to this email chain when I inquired about the reason for the delay and the exact time when they will release my relieving letter.