Got the answer: need to insert series( before RSI(

Code
	vars underlyingPrices = series(price());
	vars underlyingRsi = series(RSI(underlyingPrices, 14));

	if(!NumOpenShort && !is(LOOKBACK) && (underlyingRsi[0] < 25) && ((price(0) - price(5))/price(5) <= -0.10)) {