Hello aspirant

Hope you are doing great there . If your class 12th mark sheet is not yet released , then you have to leave that column blank that is you can not enter any other document to replace the document asked . If the form is not accepting a blank column or this is displayed as a mandatory column then I would suggest you to talk to query centre of the conducting body of  JEE Advance .

I hope my answer helped you out

Wish you all the very best !