Hi

No you are not eligible for filling JEE mains 2021 application form.

This is because,

You have passed your class 12th board examination in 2018 but One is eligible for giving JEE mains only for 3 consecutive years from the year of passing class 12th board examination so if you have passed in 2018 , you were eligible for JEE mains 2018 ,2019 and 2020 only and thus you are not eligible for JEE mains 2021

As

Eligibility criteria says that to be eligible  for JEE mains 2021 for BE or Btech you should be either appearing for 12th board exam or equivalent  in 2021 or should have passed it in 2020 or 2019 with Physics,  mathematics and anyone of Chemistry or Biology or Biotechnology or Technical Vocational Subject.

Thus, you cannot get admission in NIT as you are not eligible for JEE mains 2021 and it doesn't matter even if you have given your improvement examination in 2019 as your passing year is 2018

For detailed information go through

https://engineering.careers360.com/articles/jee-main-eligibility-criteria

Thankyou