Value prompt is used for User Selected Values which can be
qualified on datatype based on date.

make a applysimple filter that calculates the start date as 
the sysdate - the n days (here 10) and end date as sysdate.

this will give you the data related to sales of last n 
dates.

create a required value prompt on date attribute ,and
accordingly drag it in your report and save it.

You can do it without using value prompt with below steps. 
Create a filter on day attribute qualified on ID operator > 
and set value to resolve dynamically today – last 10 days.