See, BITSAT allows Candidates from last year only.
If you want to appear IN BITSAT in 2019 then you should have passed your 10+2 exam in either 2018 or 2019.
Hope this helps you.
All the best.
As far as i know if you didn't appeared for BITSAT after 12th board exams then you have 1 chance left to appear for BITSAT engineering entrance exam i.e in the next successive year nad also you need to have a minimum aggregate marks of 75% in pcm in class 12th board exams along with 60% in each subject.