I try to perform a trading operation MetaTrader 4 shows «The trade flow is busy».

A message « The trade flow is busy » appears in case of unsuccessful attempt to perform a transaction (for instance: a client sends the second order while he hasn´t received the processing results of the first one), that may occur due to a temporary loss of server connection. In such case the order is put on the queue at the client´s terminal and the above mentioned message will appear if you try to perform any operation. To clear the request queue, you should restart the terminal.