You cannot use #bench in a ranking system. Ranking system don't have the ability to resolve what #bench is, which depends on what system is using the ranking system, like a screen, or portfolio strategy, etc.
Try specifying the series parameter like this:
HighPct(251, $SP500)
or HighPct(251, GetSeries("SPY:USA"))
The second one is if you want to include dividends. For a complete list of series IDs, see MISC→TIME SERIES IDS.