Dear student,.

If you will pass your diploma this year, then yes, generally you can appear in the LEET exam 2021, for getting admission into btech 2nd year.
But you must mention the college name or examination name that for which college you will give the lateral entry entrance exam, because in some leet exams, the required eligibility is - a candidate must be passed his/her diploma equivalent exam. While some LEET exams have eligibility criteria as - candidate must be passed / appearing for final equivalent exam of diploma.

But in general if you will pass your diploma in year 2021, then you can apply for leet 2021 exam.