Hello , 

As your query is you applied for the entrance exam of engineering IIT JEE , based on as what it seems from your question. As you didn't get any college till 6th round of counselling and you were allotted the seat of IITS in 7th round of counselling and you choose to leave that seat .

Now I would like to tell you that of course you can apply for the entrance exam next year. But in any case even if it is other entrance exam apart from IIT JEE even, if you didn't get any college in the rounds of counselling. Then you have no option left with, you have to again re appear for the exam and then pass all the cases to make yourself eligible for rounds of counselling. 

If you left the counseling then doesn't allow you to appear for the advance exam directly, you have to clear the basis entrance exam.

Further, if you have any other query regarding the same you can let me know in the comment section below.

I hope your query is solved.

Thank you.