how do i show statistics in a tableau?

EDIT (based on question update):

sum(
    if (    QUARTER([Date]) = QUARTER(DATEADD('quarter',-1,TODAY()))
        and YEAR([Date]) = YEAR(DATEADD('quarter',-1,TODAY()))
       ) then [Value]
    end
)

If you’re using Tableau version older than 2019.3, you have to replace QUARTER with:

datepart('Quarter', [Date])