Use-daily-data-of-another-symbol-in-intraday-backtest

Probably your error comes from not having intraday data for the _term symbol. The script attempt to access it in the scale of the primary BarHistory.

There are at least 3 ways to do this, but in a C# Coded Strategy, the WLHost GetHistory call for Daily bars and then synchronizing is the way to go, imho.

CODE:
using WealthLab.Backtest;
using System;
using WealthLab.Core;
using WealthLab.Data;
using WealthLab.Indicators;
using System.Collections.Generic;

namespace WealthScript7 
{
    public class MyStrategy : UserStrategyBase
    {
      //create indicators and other objects here, this is executed prior to the main trading loop
      public override void Initialize(BarHistory bars)
      {
         _dailySync = WLHost.Instance.GetHistory(_term, HistoryScale.Daily, DateTime.MinValue, DateTime.MaxValue, 0, null);
         _sma200 = SMA.Series(_dailySync.Close, 200);

         //synchronize to intraday chart
         _dailySync = BarHistorySynchronizer.Synchronize(_dailySync, bars);
         _sma200 = TimeSeriesSynchronizer.Synchronize(_sma200, bars);

         //plotting
         PlotTimeSeriesLine(_dailySync.Close, _term + " Daily Close", "Price"); 
         PlotTimeSeriesLine(_sma200, _term + " Daily SMA(200)", "Price"); 

      }

        //execute the strategy rules here, this is executed once for each bar in the backtest history
        public override void Execute(BarHistory bars, int idx)
        {
            if (!HasOpenPosition(bars, PositionType.Long))
            {
                //code your buy conditions here
            }
            else
            {
                //code your sell conditions here
            }
        }

      //declare private variables below
      BarHistory _dailySync;
      TimeSeries _sma200; 
      string _term = "QQQ"; 

    }
}