Sub Or Function not Defined ('Worksheet' not working as intended)

Its been a long time since i tried VB in excel and i followed this ladies tutorial on youtube (Use Barcodes in Excel to track in and outs - Barb Henderson, 2019 [Youtube]). I copied everything and adjusted it to fit my sheets but I don't know why this error keeps occurring on this line.
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Sub checkin()
Dim barcode As String
Dim rng As Range
Dim rownumber As Long


barcode = Worksheet("Sheet4").Cells(1, 2)

    Set rng = Sheet1.Columns("a:a").Find(What:=barcode, _
    LookIn:=xlFormulas, LookAt:=xlWhole, SearchOrder:=xlByRows, _
    SearchDirection:=xlNext, MatchCase:=False, SearchFormat:=False)
    
    If rng Is Nothing Then
        ActiveSheet.Columns("a:a").Find("").Select
        ActiveCell.Value = barcode
        ActiveCell.Offset(0, 1).Select
        ActiveCell.Value = Date & " " & Time
        ActiveCell.NumberFormat = "d/m/yyyy h:mm AM/PM"
        Worksheets("Sheet4").Cells(1, 2) = ""
    End If
End Sub
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The error I am getting is "Compile error: Sub or Function not defined" It then highlights the word Worksheet in the yellow line.


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