Creating a Custom Field and Adding Calculations to a Table

Hi! I just started to watch the course video under Tableau part. I found out that I did not have the auto generated 'number of records' measurement, thus making it impossible for me to calculate a new field 'revenue' which need to multiple the 'number of records' and 'price'. I tried to multiple 'price' by 'Data(Count)' which I believe is the most similar measurement I could find in my tableau interface. However, [Price]*COUNT([Data]) shows a error of 'Cannot mix aggregate and non-aggregate arguments with this function.  Hope my description is clear. Thank you and look forward to your reply!  Best, Wenhao