Your obc-ncl certificate must be dated after 1st April 2019(including this date), otherwise you are not elible for reservation benefits. So, apply for this in start of April month and you received it after 40days harldy.

If you failed to get your certificate at time of registration of jee advance, then you can fill 'declaration in lieu of OBC NCL certificate' (available on official website)and submit it.

You should also ensure that your certificate formate is valid; accordingly as prescribed on jee advance official website. This can also cause a trouble, as face by my senior. Because his obc-ncl certificate was in Hindi formate, which is not valid for jee advance.

Best of luck for your mains and advance!!!