As per your appointement letter what is the notice period you have to serve?
Have completed your notice period?
Is there any reason behind not accepting your resignation?
Based on these question
If you are completed your notice period then ask the HR / your reporting manager to acceptence of your resignation.
If you are not yet completed your notice period just complete it.
There might be any particular reason behind not accepting the resignation.
Hope for the best if you have any other query pl feel free to ask.