Error for VBA function - "Argument Not Optional?"

Your Function is looking for a lineid as a LONG number

You are defining LineID = Dvalue = busbarArray(busbarIterator) So what is that?

How is datasheetname being defined?

All your function does is return the lineID if it's found; I'd have thought it would return the line number. Either way, the code is looking on the active worksheet, not in dataSheetName.

I would code it like this to return the row number:

Sub EG()
  Dim lineID As Long
  
  lineID = 42
  
  If findCoordinatesForX(lineID, Worksheets("Bob")) <> 0 Then
    ' whatever
  Else
    ' whatever else
  End If
End Sub

Function findCoordinatesForX(lineID As Long, wks As Worksheet) As Long
  Dim cell As Range
  
  Set cell = wks.Range("A:A").Find(What:=lineID, LookIn:=xlValues, LookAt:=xlWhole)
  If Not cell Is Nothing Then findCoordinatesForX = cell.Row
End Function

Or this to just return True if the value is found:

Sub EG()
  Dim lineID As Long
  
  lineID = 42
  
  If findCoordinatesForX(lineID, Worksheets("Bob")) Then
    ' whatever
  Else
    ' whatever else
  End If
End Sub

Function findCoordinatesForX(lineID As Long, wks As Worksheet) As Boolean
  Dim cell As Range
  
  Set cell = wks.Range("A:A").Find(What:=lineID, LookIn:=xlValues, LookAt:=xlWhole)
  findCoordinatesForX = Not cell Is Nothing
End Function