Try this code in the Sheet1 module:
Private Sub Worksheet_Change(ByVal Target As Range)

    'If column L (12) has just been entered and cells A-L on the same row are populated then sort all the data by descending column C
    
    If Target.Column = 12 And Application.WorksheetFunction.CountA(Range("A" & Target.Row & ":L" & Target.Row)) = 12 Then
        Cells.Sort Key1:=Range("C1"), Order1:=xlDescending, Header:=xlYes, OrderCustom:=1, MatchCase:=False, _
        Orientation:=xlTopToBottom, DataOption1:=xlSortNormal
    End If
    
End Sub