Cumulative calculation with Calculated column

Hi arun0386 ,
Now that you're able to retrieve week 34, this code should definitely work:

Step.01.DrillDown = 
VAR _LastWeek = CALCULATE( MAX('Table'[Week]), FILTER( ALL('Table'), NOT(ISBLANK('Table'[Inventory]))) )
RETURN
IF( 
    // checks to see if I have an inventory week
    SELECTEDVALUE('Table'[Week]) > _LastWeek, 
    // if not, let's "fill down" and return the value for the last week
    CALCULATE(SUM('Table'[Inventory]), KEEPFILTERS('Table'[Week] = _LastWeek), ALL('Table')), 
    // if there is, return me normal inventory value
    SUM('Table'[Inventory])
)

Sharing the pbix that I used for your example so you can take a look at it as a reference.

window-function-previous-row.pbix (link 1)

window-function-previous-row.pbix (link 2)