Executar ordem pendente

  Estou com problema para compilar um EA com uma ordem pendente aparece o seguinte erro " acess to non-static member or function.

Alguem poderia me ajudar?

//+------------------------------------------------------------------+
//| Expert initialization function                                   |
//+------------------------------------------------------------------+
 #include <Trade\Trade.mqh>

CTrade ctrade;

MqlTradeRequest   m_request;         // request data
MqlTradeResult    m_result;          // result data
MqlTradeCheckResult m_check_result;  // result check data

input double Lts=1;         // Lote Padrão
input double SLC=110;         // StopLoss compra
input double TPC=150;         // Take Profit compra
input double PRICE=55010;         // Preço
input long magic_number=2222;
int Buy;


int OnInit()
  {
//---
   ChartSetSymbolPeriod(0,NULL,PERIOD_M1);
   Buy=0;
//---
   return(INIT_SUCCEEDED);
  }
//+------------------------------------------------------------------+
//| Expert deinitialization function                                 |
//+------------------------------------------------------------------+
void OnDeinit(const int reason)
  {
//---
  }
//+------------------------------------------------------------------+
//| Expert tick function                                             |
//+------------------------------------------------------------------+
void OnTick()
  {
//---
      //---------- Condição  compra----------------------------
   bool compra_ts = (Buy<1);         // 
   if(compra_ts)
     {
        CTrade::OrderOpen(_Symbol,ORDER_TYPE_BUY,Lts,PRICE,PRICE,SLC,TPC,ORDER_TIME_GTC,0,"Ordem de compra");
      Buy=1;
        }
  }