Mop up round is conducted for admission to Deemed/Central Universities and IP Quota of ESIC Colleges. You haven't mentioned whether you have qualified in NEET MDS or not. This is important because even to participate in NEET MDS Mop up round, you have to qualify in PG Dental entrance exam by securing the minimum qualifying percentile. The cut offs for various Categories for NEET MDS 2020 have been as follows:

General Category: 50 percentile: 286

OBC/SC/ST: 40 percentile: 250

General: PWD: 45 percentile: 268

OBC/SC/ST: PWD: 40 percentile: 250

If you have scored equal to or more than this, you can register for NEET MDS and fill in the choices. But if you haven't scored the required marks, then you cannot register for the Mop up round because you did not qualify in NEET MDS 2020. You might have to reappear for 2021.