Some common reasons why an order is not accepted can include (but is not limited to):
Price Limit: Many exchanges set a range of acceptability and will reject an order in the case that the order price exceeds this band. This is to prevent excess volatile movements in price of securities. As such, an order can be rejected in the case that the limit price is too high or too low compared to last traded price.
Worst Case Execution Scenario: If your market order has been rejected this may be because the worst case execution scenario exceeds your available to trade amount. As the price of a market order is not set until the order is executed, we must account for a sudden change in price in the time from the order being placed to the time of execution. If this seems to be the issue then you may wish to either reduce the quantity of the market order or place a limit order, where the maximum execution price will be less than your available to trade.
Incorrect Stop Price: Stop Loss and Stop Limit orders are intended to trigger at a certain point after a fall in stock price. As such, when entering a Stop price, it must be lower than the current trading price of the security.
Tick Size: Some products set a fixed tick size for the minimum movement in the price. An order may be rejected if this price limit is not met. For instance; if a stock requires all prices to be set to a tick size to the nearest €0.02, an order for €0.35 would be rejected. In essence, the limit price set must be divisible by the tick size.
Lot Size: Some exchanges require stocks to be purchased in allotments of certain amount such as 1,000, 10,000, etc. This is particularly common in Asian markets.
DEGIRO suggests familiarising yourself with the rules and conditions of the exchange before you start trading.