1. Typo -- see screenshot
Capture.JPG
2. Not sure about the rest of your logic
3. Not sure why you wanted to use Index. I'd use VLookup and simplify
Option Explicit
Sub IndexMatch()
Dim wsDestination As Worksheet, wsPopulation As Worksheet
Dim n As Long
Dim rPopulation As Range, rDestination As Range
Set wsDestination = ThisWorkbook.Worksheets("Destination")
Set wsPopulation = ThisWorkbook.Worksheets("Population")
Set rPopulation = wsPopulation.Range("A:B")
Set rDestination = wsDestination.Cells(1, 1).CurrentRegion
For n = 2 To rDestination.Rows.Count
On Error Resume Next
wsDestination.Cells(n, 3).Value = Application.WorksheetFunction.VLookup(wsDestination.Cells(n, 1).Value, rPopulation, 2, 0)
On Error GoTo 0
Next n
End Sub