Hello,
Yes you can drop a year for JEE Mains. Please note that, if you have appeared for your class 12 in the years 2019 or 2020, then only you will be eligible for JEE Mains 2021. Jee. Mains allows three consecutive years for JEE Mains.
If you drop a year for JEE Mains, you lose one-year of your study life. Your friends might be one year ahead of you. But considering the bright side, you get one extra year of preparation. If you consistently keep on working hard and practice a lot, then you might get top IITs and NITs.

So as you are dropping a year, please don't lose focus and always stay motivated. You need to follow the best books for JEE Mains and for JEE Advanced. Also practice a lot of questions and a lot of Mock tests. Also please practice the previous year question papers so that you gain an insight into the real exam.
Please use the link to get to know about the detailed preparation strategy for JEE mains https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/jee-main-preparation-tips/amp&ved=2ahUKEwjM5rmy58TrAhWiwjgGHRmRDvkQFjALegQIAhAC&usg=AOvVaw3GSa8oPTbvFl7CX28keJA1&ampcf=1

Hope it helps

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