Yes, you can apply for Computer Science subject in NET exam, even if you have done MBA in Financial Management. So if you have B.Tech in Computer Science, you will not have any problem in giving NET.

Regarding LS certificate: If you apply on the basis of your B.Tech, you will not get LS certificate, because this certificate is given only to those candidates who have done Master's degree. So if you apply on the basis of MBA, you can get LS certificate, but you have to see whether your Master's degree is related to Computer Science subject or not.