I need to set a custom date field (in header)with the value of trandate + 180 days. for example, if trandate is 21/03/2022 then the custom field value should set as 17/09/2022. I tried doing through workflow set field value action, i tried {trandate}+180 but its not working. I found one field formula ADD_MONTHS(date, integer) but this also not working. Can anyone help me in doing this?

Do you have the Store Value box checked? When you use a formula to calculate results this needs to be unchecked.