Hello Shivang Desai,
This calculation can be done with a replacement based on a condition. So create a new version of [SameWeekLastyear] based on the condition of Currrent week being Null. Make sure the calculation is happening per row so there should be no aggregation in the calc.
If isnull([Current Week Net]) then Null else [Same Week Last Year Net] END
Then you can do your calculation.
(sum([Current Week Net])/sum([fixed same week last year])-sum([fixed same week last year]))*100
Atttached is a workbook example.
I hope this helps