The Backtest chart creates an instrument that is only used to plot the account balance information.  The "not marked intraday" message is not relevant here.  You can disregard.  That is important for symbols for which you are tracking live.  Those symbols that you are tracking live and marked for intraday by default so not something you need to be concerned with.  The backtest symbol is not a symbol that will be tracked live so not marked intraday.