This might put you back on track:

Code:
// PRUEBA DE MULTIPLICADOR ////////////////

var multiplier=0.0;

function run() 
{
	StartDate = 20160101; // 20160101
	//EndDate = 20170301;
	EndDate = 20181231;
	BarPeriod = 5;	// 1 hour
	LookBack = 200;

	asset("EUR/USD"); //"EUR/USD"
	
	
	static var multiplier=1;

	vars Price = series(priceClose());
	
	var myRSI = RSI(Price, 100);
	
	static int neverplotted = 1;
	
	if(!is(LOOKBACK) && neverplotted) 
	{
		neverplotted = 0;
		printf("nmyRSI = %.1f, multiplier before set = %.1f, priceClose() = %.5f", myRSI, multiplier, priceClose());
		multiplier=priceClose()/myRSI;
		printf("nmultiplier after set = %.5f", multiplier);
	}
	
	//var multiplier=priceClose(0)/myRSI[0]; this one would change multiplier value at every new bar, so delete it.
	vars myPrice=series(myRSI*multiplier);
	
	//if(!is(INITRUN)) printf("nmultiplier = %.5f, Price[0] = %.5f, myRSI = %.0f, myPrice[0] = %.5f", multiplier, Price[0], myRSI, myPrice[0]);
	

// plot signals and thresholds

   plot("myPrice", myPrice, 0, RED);
	
	PlotWidth = 600;
	PlotHeight1 = 300;
	set(PLOTNOW);
}