Aastha, you need to be 17 years atleast by December 31st of the year of admission. So, if you want to appear for NEET 2020,  you should be 17 years atleast by December 31st, 2020. But if you have been born in 2004, you will be 16 years by December 31st, 2020 and not 17. So you can only appear for NEET 2021.

As per the criteria given by the NTA, you are not eligible to write NEET exam this year (2020). But you will be eligible to write NEET exam next year  (2021). Keep preparing for the exam for the next year. You will definitely perform good in the exam next year.