Now, i created a workflow to set the Case Due Date = start date + target open days but it doesn't work also I tried this formula nlapiDateToString(nlapiAddDays(new Date({startdate}), parseInt({custevent_case_due_date}))) but I get an error 'The date format must be M/D/YYYY' but I the date format is already in this format

{trandate}+2 That'll add 2 days to the transaction date so just use your fields there NVL({start_date_field_id},0) + NVL({your_other_field_id},0)

This is not working. I didn't get any value in the Due date