Non puoi mettere BUY/SELL in un indicatore, ma in una strategia.

Gli indicatori NON possono acquistare o vendere.

Adesso è chiaro, ho aggiunto le tue righe, modificate, alla fine del codice, immediatamente prima di RETURN:

//PRC_Pivot Point Supertrend | indicator
//30.06.2020
//Nicolas @ www.prorealcode.com
//Sharing ProRealTime knowledge
//converted from pinescript

// -- settings
prd = 2 //Pivot Point Period
Factor = 3 //ATR Factor
Pd = 10 //ATR Period
showpivot = 0 // Show Pivot Points (1 = true ; 0 =false)
showcl = 0 //Show PP Center Line (1 = true ; 0 =false)
showsr = 0 //Show Support/Resistance (1 = true ; 0 =false)
// --- end of settings

atr = averagetruerange[14]
if high[prd] >= highest[(prd)*2+1](high) then
ph = high[prd]
if showpivot then
drawtext("H",barindex[prd],ph+atr/2,dialog,standard,20) coloured(255,0,0)
endif
lastpp = ph
endif

if low[prd] <= lowest[(prd)*2+1](low)  then
pl = low[prd]
if showpivot then
drawtext("L",barindex[prd],pl-atr/2,dialog,standard,20) coloured(0,255,0)
endif
lastpp = pl
endif

if lastpp<>lastpp[1] then
if center=0 then
center = lastpp
else
center = (center * 2 + lastpp) / 3
endif

endif
Up = center - (Factor * averagetruerange[Pd])
Dn = center + (Factor * averagetruerange[Pd])

Trend = 0
if close[1] > TUp[1] then
TuP = max(Up, TUp[1])
else
TuP= Up
endif
if close[1] < TDown[1] then
TDown= min(Dn, TDown[1])
else
TDown= Dn
endif
if close > TDown[1] then
trend = 1
elsif close < TUp[1] then
trend = -1
else
trend = Trend[1]
endif

once trend=1
if Trend = 1 then
trailingsl =  TUp
r=0
g=255
else
trailingsl = TDown
r=255
g=0
endif

if showcl then
alpha=255
if center<medianprice then
clr=0
clb=255
else
clr=255
clb=0
endif
endif

sr = undefined
if showsr then
sr = lastpp
if lastpp=ph then
srr=255
srg=0
else
srr=0
srg=255
endif
endif

MyLongConditions = close CROSSES OVER trailingsl
MyShortConditions = close CROSSES UNDER trailingsl
IF MyShortConditions THEN
DrawText("Sell",barIndex, trailingsl + range,serif,bold,15) coloured(255,0,0,255)
ELSIF MyLongConditions THEN
DrawText("Buy",barIndex, trailingsl - range,serif,bold,15) coloured(0,128,0,135)
ENDIF

return trailingsl coloured(r,g,0) style(line,2) as "Supertrend pivot point", center coloured(clr,0,clb,alpha) as "early exit", sr coloured(srr,srg,0) style(point,2) as "support / resistance"