I Am taken admission in NIT in 2019&i passed my 12th exam in 2018& I attempt in jeemain in 2018&2019 but I attempt jeead only in 2019 then am I eligible for jeead 2020?

Hello,
As you have passed your class 12 in 2018 so you can only appear for Jee advanced in the two consecutive years 2018 and 2019. Even though you didn't appear for Jee advanced 2018 and appeared for Jee advanced 2019, you are still not eligible for Jee mains 2020. But you can appear for Jee mains 2020 for the last time. So you must prepare well for Jee mains 2020 if you are going to appear for it. But as you are already in an NIT so I don't think you need to appear for it again. No matter what you cannot get into IITs for undergraduate course now.
Hope it helps