No. Computer Science subject in 12th is not must. As per the eligibility criteria for DU  Bsc Computer Science, Candidates should have Mathematics, one language and two other elective subjects as listed academic subject obtaining 55% marks or securing at least 60% marks in Mathematics. Candidates who have studies Physics, Chemistry/Computer Science and Mathematics will be given a 5% of advantage in the ‘Best of Four’ calculation.

So if you meet the above criteria then you are eligible.