Runtime Error 5, invalid procedure or call argument - vba

Question

I am trying to assign Cell E8 in Sheet"Report" with an Index Match formula with a dynamic range. The range is from Sheet"Data"
I have found the last row (LR) and last column (lc).
The run time error occurs at the last line: Cell("E8").formula = "=...."
This is the code:
Sub report()
Dim LR As Long, lc As Long, first As Long, proxy As String
Sheets("Data").Select
'Finding the first filled cell by moving down from A1
first = Sheets("Data").Range("A1").End(xlDown).Row
'The first row has column headers: Name, ID number, etc... SO I assign it to the next row where the first data entry is
first = first + 1
LR = Sheets("Data").Range("A" & first).End(xlDown).Row
lc = Sheets("Data").Range("A" & first).End(xlToRight).Column
Sheets("Report").Select
proxy = "=IFERROR(INDEX(Data!$A$10:" & Cells(LR, lc).Address & ",MATCH(Report!$C$3,Data!$A$10:" & Cells(LR, 1).Address & ",0),MATCH(Report!$C8,Data!A$9:" & Cells(9, lc).Address & ",0)),'N/A')"
Cells("E8").Formula = proxy
End Sub

Answer 1

Sub report()
Dim LR As Long, lc As Long, first As Long, proxy As String
With Sheets("Data")
'Finding the first filled cell by moving down from A1
first = .Range("A1").End(xlDown).Row
'The first row has column headers: Name, ID number, etc... SO I assign it to the next row where the first data entry is
first = first + 1
LR = .Range("A" & Rows.Count).End(xlUp).Row
lc = .Range("A" & first - 1).End(xlToRight).Column
End With
proxy = "=IFERROR(INDEX(Data!$A$10:" & Cells(LR, lc).Address & ",MATCH(Report!$C$3,Data!$A$10:" & Cells(LR, 1).Address & ",0),MATCH(Report!$C8,Data!A$9:" & Cells(9, lc).Address & ",0)),""N/A"")"
Sheets("Report").Range("E8").Formula = proxy
End Sub

Answer 2

You are using single quotes to wrap 'N/A'. You need double quotes and because they are quotes within a quoted string, you need to double them up. Additionally, the Range.Cells property does not accept the same style of cell address references that a Range object does.
proxy = "=IFERROR(INDEX(Data!$A$10:" & Cells(LR, lc).Address & _
",MATCH(Report!$C$3,Data!$A$10:" & Cells(LR, 1).Address & _
",0),MATCH(Report!$C8,Data!A$9:" & Cells(9, lc).Address & _
",0)),""N/A"")"
Sheets("Report").Select
Range("E8").Formula = proxy
Here is a quick rewrite that gets away from using Worksheet.Select¹ method and the implicit ActiveSheet property.
Sub report()
Dim lr As Long, lc As Long, first As Long, proxy As String
With Worksheets("Data")
'Finding the first filled cell by moving down from A1
first = .Range("A1").End(xlDown).Row
'The first row has column headers: Name, ID number, etc... SO I assign it to the next row where the first data entry is
first = first + 1
lr = .Range("A" & first).End(xlDown).Row
lc = .Range("A" & first).End(xlToRight).Column
End With
proxy = "=IFERROR(INDEX(Data!$A$10:" & Cells(lr, lc).Address & _
",MATCH(Report!$C$3,Data!$A$10:" & Cells(lr, 1).Address & _
",0),MATCH(Report!$C8,Data!A$9:" & Cells(9, lc).Address & _
",0)),""NA"")"
With Worksheets("Report")
.Range("E8").Formula = proxy
'alternate with .Cells as one of these
'.Cells(8, "E").Formula = proxy
'.Cells(8, 5).Formula = proxy
End With
End Sub
.¹ See How to avoid using Select in Excel VBA macros for more methods on getting away from relying on select and activate to accomplish your goals.