A Cash account must have enough cash to cover the cost of a position plus commissions and can never hold a negative cash balance. When placing a Market order (or Stop order), the order value cannot be known with certainty prior to execution as the market can move in any direction at any moment.
Because of this, an additional 5% is added to the order amount at the credit check step for Market type orders to account for market movements. If you receive an order rejection for not enough Settled Cash when you do in fact have the Settled Cash to execute your order at your desired price and quantity, try using a Limit order (or Stop Limit order for a Stop) or reduce the MKT order size.
Clients may also receive this rejection message when attempting to purchase a foreign security in a Cash type account. Because a Cash account is precluded from borrowing funds, clients will need to place a currency conversion and wait for those funds to settle prior to purchasing a foreign security.
More details regarding: closing the nominal forex balances in an account may be found in the MEXEM Knowledge Base.
See also: How do I upgrade my account to use margin or Portfolio Margin?