Hello Bhavana,
Yes You have tables to find the certification Details of Notification.
There is a Table called DIACL_DS which has a field named as SIGN_ID, the where condition parameter for this table is object number of notification which you have to feed onto the field OBJ_ID of the table. This SIGN_ID will get generated only when the notification is certified, if there is no entry then it means the notification is not certified.
Also there is a table called SIGNS where you can enter sign id and get the certified username certified time from fields SIGN_SIGNER and SIGN_TSTAMP respectively.
However using the GET_CERTIFICATION method is the shortest approach and effective to use. I think there should be some role assignment issues with your ID. try with different user IDs who have higher roles than you.
Please try using the same method by figuring out what went wrong. Use these tables if you are not able to use the method.