The reason why you cannot place a pending order may be incorrect S/L or T/P values. Besides, you need to make sure that you are setting the correct price level at which your order should be triggered. Here are the main types of pending orders in MT5:
Buy Limit - an order to buy at the Ask price that is equal or lower than specified in the order
Buy Stop - an order to buy at the Ask price that is equal or higher than specified in the order
Sell Limit - an order to sell at the Bid price that is equal or higher than specified in the order
Sell Stop - an order to sell at the Bid price that is equal or lower than specified in the order
Buy Stop Limit - a combination of Buy Limit and Buy Stop orders. It is intended for placing a Buy Limit order as soon as the Ask price reaches a predetermined level
Sell Stop Limit - an order to place a Sell Limit as soon as the Bid price reaches a predetermined level