Since at the time of filling up the form, you applied as general category you will considered as a general category candidate only. Nothing can be done now. Hope this helps!

Hii Neeraja

Since you have filled the form as General category, you won't be able to show your caste certificate in your entire selection process and your scores would be considered as per the general category marking procedure only.

You can contact on the helpline number or mail it at the official site.However, not really sure, if this would be resolved or not  

Yes, it will be problematic during admission because your scores will be consider with general category only, you cannot show your caste certificate later while taking admission it could be considered as forgery or false certificate.

In order to save yourself from the problem, you should mail this error information to the official CAT site addressing your problem and also attach your OBC caste document with it. You will also need to give proof that you have not made this document after giving the exam because it might not be considered valid. Also, you are very late, you could have direct mail this error problem after filling the form and it would have been rectified even before your exam. I am not sure whether you will now even get a response from CAT administration or not.