Excel VBA For Creative Problem Solving Part 1 Assignment 4

Hi all,
Can someone to provide me with a to solve the assignment #4? My subroutine as follows:

Sub HighlightRows()
Dim nr As Integer, i As Integer, identifier As String, key As Integer, ID As String
nr = WorksheetFunction.CountA(Columns("A:A"))
For i = 2 To nr
   ID=Cells(i,1)
   If identifier(Range(ID) = Range("identifier") And key(Range(ID) =Range("key") Then Range("A:C" & i).Interior.ColorIndex = 4
   End If

Next i

End Sub

Not sure why "expected array" error pops up.

Please, I just want an "advice" , not a complete fix.

Thanks in advance
PS: fairly new with VBA by the way