So, if you could not be in top 20% percentile of your board in the year 2019 and then if you appear again in the year 2021 for JEE Main and board exams, then it will not create a problem in JEE Main and admission to NITs but after clearing JEE Main, if you wishes to appear for JEE Advanced and to take admission in IITs then you will not be able to do so. Because JEE Advanced criteria does not allow anyone to do so.

Please be clear what you meant to say by "Board is not inproment new section in up board so any problem in counseling time in nit " and do let me know whether your query is resolved or not.