Save File with TimeStamp

Hi All, I've got the below code which gives me "Type mismatch" on 'CStr'. Would you be so kind please and advise as to how I can save this file so that it follows the outlined format below?

    Dim wb As Workbook

    Dim timeStamp As String
    Dim filePath As String
    Dim fileName As String
    filePath = ThisWorkbook.Path
    timeStamp = format(CStr(Now), "dd.mm.yy hh:mm")
    fileName = "Test - " & Worksheets("MainTab").Range("E4").Text & " - " & timeStamp

    wb.SaveAs fileName:=filePath & "\" & fileName & ".xlsx", ReadOnlyRecommended:=True

Many thanks!