Variance % selected week- DAX

Hi, I am not sure how your semantic model looks like, but I tried to create a sample pbix file like below.

Please check the  below picture and the attached pbix file.

INDEX function (DAX) - DAX | Microsoft Learn

Sales measure: = 
VAR _sales =
    SUM ( Data[Sales] )
VAR _weekselectcondition =
    COUNTROWS ( DISTINCT ( Week[Week] ) ) = 2
VAR _totalcolumn =
    NOT ( HASONEVALUE ( Week[Week] ) )
VAR _salesone =
    CALCULATE (
        SUM ( Data[Sales] ),
        INDEX ( 1, VALUES ( Week[Week] ), ORDERBY ( Week[Week], ASC ) )
    )
VAR _salestwo =
    CALCULATE (
        SUM ( Data[Sales] ),
        INDEX ( 1, VALUES ( Week[Week] ), ORDERBY ( Week[Week], DESC ) )
    )
VAR _percentage =
    FORMAT ( DIVIDE ( _salestwo - _salesone, _salesone ), "#0.00%" )
RETURN
    IF ( _weekselectcondition && _totalcolumn, _percentage, _sales )