'Error 1004' when concatenating string - excel

Question

I've concatenated a string to paste an index match formula into rows of a column. Every time I try running this piece of code, I get Runtime Error '1004, but I can't see what I have wrong. Here's the code I have:
Dim j As Long
'Loop down the rows in mainfile
For j = 2 To lastFullRow2
Dim firstArgument As String
firstArgument = "Sheet2!" & valuecolumnLetter & "2:" & valuecolumnLetter & lastFullRow1 & ""
'MsgBox "firstArgument" & firstArgument
Dim secondArgument As String
secondArgument = "Sheet2!" & parameter1columnLetter & "2:" & parameter1columnLetter & lastFullRow1 & ""
'MsgBox "secondArgument " & secondArgument
Dim thirdArgument As String
thirdArgument = "Sheet2!" & parameter2columnLetter & "2:" & parameter2columnLetter & lastFullRow1 & ""
'MsgBox "thirdArgument " & thirdArgument
Dim fourthArgument As String
fourthArgument = "Sheet2!" & parameter2columnLetter & "2:" & parameter2columnLetter & lastFullRow1 & ""
'MsgBox "fourthArgument " & fourthArgument
Dim condition3 As String
condition3 = "Sheet3!" & "D2:" & D & j & ""
'MsgBox "condition3 " & condition3
Dim patid1 As String
patid1 = "Sheet2!" & "D2:" & D & lastFullRow2 & ""
'MsgBox "patid1 " & patid1
With ws_mainfile
Dim commandstring As String
commandstring = "=INDEX(" & firstArgument & ",MATCH(1,(" & secondArgument & "=" & condition1 & ")*(" & thirdArgument & "=" & condition2 & ")*(" & patid1 & "=" & condition3 & "),0))"
ws_mainfile.Range("AN" & j).FormulaArray = commandstring
End With
Next j
The debugger is saying the error is at the ws_mainfile.Range... = commandstring line.

Answer 1

condition3 = "Sheet3!" & "D2:" & D & j & ""
patid1 = "Sheet2!" & "D2:" & D & lastFullRow2 & ""
Have you defined a variable D and what is its value?
Maybe you meant:
condition3 = "Sheet3!" & "D2:D" & j
patid1 = "Sheet2!" & "D2:D" & lastFullRow2
There's also no need to concatenate an empty string onto the end of those lines.