dear sir i cleared my class 12 throuigh nios this year 2020 with only 54%, i am planning to sit on improvement exam in 2021 will i be eligable for jee 2021(mains/advanced)?

Hello,
Yes, you are obviously eligible for JEE 2021 mains and advanced.
These are the age criteria for it according to jee main site
"For appearing in the JEE (Main) examination there is no age limit for the candidates. The candidates who have passed class 12th / equivalent examination in 2019, 2020 or appearing in 2021 irrespective of their age can appear in
JEE (Main) examination 2021."

"Candidates must note that the IITs permit only two attempts for the JEE (Advanced), i.e. a candidate should have appeared for the Class XII (or equivalent) examination for the first time in either 2020 or 2021, subject to the condition of not having been admitted at an IIT on earlier occasion. Details may be obtained from the Information
Brochure of the JEE (Advanced)-2021."

You can check the details on the link below for criteria for 2020 and applied it on 3021 and have detailed information on the examination rule of 2021.
https://jeemain.nta.nic.in/WebInfo/Handler/FileHandler.ashx?i=File&ii=256&iii=Y

Hope you got it.
Thank you