There are a variety of reasons why an order may be rejected.
While using the demo trading mode, all executions need to be performed during market hours. Most instruments can be traded from 9:30 am to 4:00 pm EST. Remember, in the demo feature that some markets may be delayed and this could also impact your demo trades.
With live accounts, you can make executions during market hours, or after hours if you possess available trading permissions. If you do not hold the necessary margin required to trade a futures contract, or to make the purchase of an equity or foreign exchange pair then you may not be able to execute your order. Please contact your broker for more detailed information on permissions and trading hours.
After hours trading in markets like Forex and Futures can transcend these rules.