Column to be modified is not an identity column

Alternative solution if you don't want to create new column:

CREATE SEQUENCE s_roll_seq
     START WITH 1 -- here last id + 1
   INCREMENT BY 1;

ALTER TABLE students
    MODIFY S_ROLL NUMBER DEFAULT s_roll_seq.NEXTVAL;

You're getting this error simply because modifying an existing column as IDENTITY column is not supported right now.

The solution is to add a new column and then drop the existing one (making sure that you do take care of the data too).