Yes you can take one year drop for BITSAT examination. But the drop can only be of one year. If you have taken more than one year drop, then you are not eligible for BITSAT as is clearly mentioned in brochure of BITSAT 2019:

"Only Students who are appearing for 12th examination in 2019 or who have passed 12th Examination in 2018 are eligible to appear in the BITSAT-2019 test. If a candidate has taken more than one attempt in 12th class or its equivalent, only his latest performance is considered, provided this attempt has been for the full component of subjects/courses prescribed. Students who have passed 12th examination in 2017 or earlier are NOT eligible to appear in BITSAT-2019."