I am also facing same problem but i am in SRM 5.0 SP7.. I already look at the answer reply by Disha oss notes.. since i am in SRMN 5.0 SP7 is not required one.
and also i am getting error message 'Approval step without ad-hoc capability.No preview possible' in the approver preview whenever I add approver by 'Add Approver' button by Approver Preview.. If I don't add approver everything is working fine..
please advise me to fix this issue..