I had nt added details of my 12th class in neet form would i be allowed to sit in exam??

What do you mean by the class 12 details if you are referring to your marks then if you have already passed your class 12 at a previous year you should I fill in the details of your marks but if you have appeared for class 12 this year and have not got your marksheet then you are not supposed to fill the marks as you cannot possibly know your marks so it won't be a problem
if you have already got your marks having appeared for the examination at a previous year you should have filled in the marks and if you have not then it is better if you would address the issue at the NBA help us so that they could solve it out for you since it could be the problem during verification of your document safe you produce your marksheet then and it is found out that you have not entered your marks since your candidature can be in question
However if you have appeared for your boards this year then it is not possible for you to know your marks so you need not panic about non entry of your class 12 details