There is a verity 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. For more information on trading delays with the demo feature on Pilot Trading click here.
With live accounts, you can make executions during market hours, or after hours if you possess available trading permissions. Contact your broker for more detailed information on permissions and trading hours.
After hours trading in markets like Forex and crypto can transcend these rules.
With crypto, you need to own one or both of the pairs involved in the trade depending on what you are buying or selling. For instance, you would need USD to buy LTC, or BTC if the pair were LTC/BTC. This means, unlike a short, you will need to own LTC or Litecoin in order to sell it. Some exchanges will off high tier account margins for cryptocurrency trading.
For more info on market hours, please click here.