Yes, Adarsh, you can definitely complete the entire syllabus of JEE Main while you are still in class XIIth, and it is a very common thing that many rankers do.

I would recommend you to first focus towards the completion of your class XII syllabus. Try to complete the class XII syllabus till the month of August (max September mid). Once you are through with you class XII syllabus, now will be time to aim to complete the class XI portion. During the month of September and October, complete the main portion of class XI like Gravitation, Fluids, NLM, Conic Sections, Equilibrium and also the scoring chapters like Chemistry in Everyday life. From November, focus on your weak areas and give as many mock tests as you can. Once you are through with this schedule, you are all set to give the first round of JEE Main.

Hope I have answered your question. All the best!