I have spent quite some time trying to get a basic volatility trading algo working using options. For some reason I have hit an error that I cannot resolve. I have attached a backtest below, any help would be appreciated.

For some reason I was not able to attach a backtest, so the code for the algo is below:

from datetime import timedelta

class OptionsAlgorithm(QCAlgorithm):

def Initialize(self):
self.SetStartDate(2017, 01, 01)
self.SetEndDate(2017, 01, 8)
self.SetCash(100000)
equity = self.AddEquity("VXX", Resolution.Minute)
self.symbol = equity.Symbol
self.last_slice = None

# warm up for 121 days
self.SetWarmUp(timedelta(121))

# filter the option contracts we will trade
option = self.AddOption(str(self.symbol));
option.SetFilter(-2, +20, TimeSpan.FromDays(5),TimeSpan.FromDays(9));

# run rebalance every day before market close
self.Schedule.On(self.DateRules.Every([DayOfWeek.Friday]), self.TimeRules.BeforeMarketClose(str(self.symbol), 2), Action(self.rebalance))


def OnData(self,slice):
if self.IsWarmingUp: return
# if self.Time.weekday() != 4 or self.Time.hour != 12 or self.Time.minute != 58: return

self.last_slice = slice


def rebalance(self):
self.Debug("rebalance being called")

# sell everything first
self.Liquidate()

for i in self.last_slice.OptionChains:
chain = i.Value

# differentiate the call and put options
calls = [x for x in chain if x.Right == 0]
puts = [x for x in chain if x.Right == 1]

# get all OTM calls
otm_calls = [x for x in calls if x.UnderlyingLastPrice - x.Strike < 0]

# get closest ATM call
atm_call = sorted(otm_calls, key = lambda x: x.UnderlyingLastPrice - x.Strike)[0]

# get OTM call at least 10% OTM
fotm_calls = [x for x in otm_calls if float(x.UnderlyingLastPrice)*float(1.15) - float(x.Strike) < 0]
fotm_call = sorted(fotm_calls, key = lambda x: x.UnderlyingLastPrice - x.Strike)[0]

# get all OTM puts
otm_puts = [x for x in puts if x.UnderlyingLastPrice - x.Strike > 0]

# get 2nd closest ATM put
put = sorted(otm_puts, key = lambda x: x.Strike, reverse = True)[1]

# sell 80% ATM calls
atm_call_amount = float(self.Portfolio.TotalPortfolioValue) * -0.8
atm_call_contracts = atm_call_amount / (float(atm_call.Underlying.Price)*100.0)
self.MarketOrder(atm_call.Symbol, atm_call_contracts)

# buy 80% Far out of the money calls
fotm_call_amount = float(self.Portfolio.TotalPortfolioValue) * 0.8
fotm_call_contracts = fotm_call_amount / (float(fotm_call.Underlying.Price)*100.0)
self.MarketOrder(fotm_call.Symbol, fotm_call_contracts)

# buy 32% puts
put_amount = float(self.Portfolio.TotalPortfolioValue) * 0.32
put_contracts = put_amount / (float(put.Underlying.Price)*100.0)
self.MarketOrder(put.Symbol, put_contracts)