Hi Hema
You have not selected the actual step which is giving the error -> that is the last step...the step history for that step will have the actual error.
PS: again note my earlier comment....if the step history of that step has an error ...something like "Object not found" ...and you are using standard mail step - BO SELFITEM method SENDTASKDESCRIPTION....then the issue is as mentioned earlier .........
It will give error if any of the entry in importing parameter addressstring is empty (even one line amongst others) ...make sure that we have atleast one recipient and there are no empty lines.
Regards,
Modak