I am trying to run LEAN using my own local data with a universe of all SP500 stocks, and observe the following error. If I reduce the number of symbols this error disappears.

Backtest Handled Error: AAPL: The security does not have an accurate price as it has not yet received a bar of data. Before placing a trade (or using SetHoldings) warm up your algorithm with SetWarmup, or use slice.Contains(symbol) to confirm the Slice object has price before using the data. Data does not necessarily all arrive at the same time so your algorithm should confirm the data is ready before using it. In live trading this can mean you do not have an active subscription to the asset class you're trying to trade. If using custom data make sure you've set the 'Value' property.

Example:

        for symbol in tickers:  #450+ symbols causes backtest error
           equities[symbol] = self.AddEquity(symbol, self.resolution, fillDataForward = True, leverage = 0, extendedMarketHours = False)

        for symbol in tickers[:350]:  #350 symbols no error
           equities[symbol] = self.AddEquity(symbol, self.resolution, fillDataForward = True, leverage = 0, extendedMarketHours = False)

I've observed the same using:      

equities = [Symbol.Create(symbol, SecurityType.Equity, Market.USA) for symbol in tickers]
self.SetUniverseSelection(ManualUniverseSelectionModel(equities))