Hello Ramya,

The JEE Advanced is the gateway examination for the nation's top-most premiere colleges, the IITs. It is only appeared by a selected sect of students from the JEE Mains. A candidate becomes eligible to register for JEE Advanced if and only if they have cleared that respective year's cutoff in JEE Mains according to their category.

In your case you have not given your Mains in 2021, you can surely appear in Advanced 2023 after qualifying the Mains 2023, so keep a high score to avoid any hassles.

Hope it helps, thanks!