Could you please attach sample data as twbx format?
This formula can be fixed as:
running_sum(sum(IF FY End=FY Year then value end))
But since you are looking for data between 2 selected years. I would suggest create a calculated field T|F as :
FY>= FY START and FY<= FY END
FY START/END I am assuming the parameters from which user will choose the years.
Then put this field on filter shelf and choose true.
After this you can calculate your running sum simply by choosing running_sum table calculation of tableau.