In which scenario HR can refuse to give acceptance to resignation?

In which scenario HR can refuse to give acceptance to resignation? what if they delayed it and employee lost his potential opportunity in another organisation .What are our rights as an employee?

HR can refuse acceptance of resignation if request is not as per terms and conditions of appointment.
In theory employee can sue the company for delayed acceptance of resignation and seek damages for proven loss of opportunity.
But in practical life, no effective remedy exists due to delays in judicial system.
Exits should be well planned, foreseeing all hurdles and delays.
Difficult balancing act will have to be done many times.

Dear Anamika, Please note acceptance/Refusal of Resignation to be done by "Appointing Authority" after due diligence.I presume you have not issued Appointment Letter.Consequently, Resignation to be submitted to the Appointing Authority for consideration as per the Terms & conditions of Appointment