Consider declaring indicators once in __init__
:
self.HighestC = bt.indicators.Highest(self.data, period=9)
self.LowestC = bt.indicators.Lowest(self.data, period=9)
and then use
self.l.Stoc[0] = (self.data[0]-self.LowestC[0])/(self.HighestC[0]-self.LowestC[0])