1. Your date formats between Processed Date and Campaign Start Date are different we need to convert them. However, your expr will show 0 or nothing coz your dates are not between the processed dates. Could be that you just gave sample data which is not matching.
2. Your sales field name is different form the data sample to that in the expression. Like in your expr you have [Sales Amt] but your sample data set has a field name Sales.
However, I have created few date ID's to use them in your set analysis please have a look if this is of any help.
Can we know why you are using P() here just wanted to understand.
I used text object to check Avg sales. Changed few Process Dates to 2015 as all of them were in 2018 which always display 0 for Avg. so changed few dates that we can get some values. Check screenshot below.