"Not enough money" means the trader does not have enough money to open a position. To open position, you must make a deposit (provide the reserve funds).
<Reserve Funds> = <Value of the contract> / <Level>
Example: The trader has 1,000 EUR on his account and he sends an order to the broker to open a position for 50 lots (5,000,000) EURUSD
The server receives the order and calculates the reserve required to open this position: 50 lots (EURUSD) at 1:100 level = 5,000,000/100 = 50,000 EUR.
Since the customer has only 1,000 EUR, and the opening of this position requires 50,000 EUR, the customer will receive a rejection from the server: "Not enough money".