Every order is checked before it leaves the platform. A rejection is the platform refusing to send something that would have broken a rule, so it is usually protecting you rather than malfunctioning.
Work down this list in order. The first match is almost always the answer.
You are locked out
A lockout blocks new exposure. It might be one you set yourself, one your firm applied, or an automatic lock from breaching your daily loss limit or daily drawdown.
The terminal shows an active lockout as a banner in the order ticket, with a countdown where one applies. If your firm applied it, a reason is shown with it.
Closing, reversing, cancelling and flattening all still work while locked.
The market is closed
Futures do not trade continuously. Outside the session, and during the daily break, orders will not go through. Your firm may also trim the session slightly at each end.
You do not have live market data yet
Live CME data requires the market-data agreement accepted, a complete profile, a linked Discord account and an active trading account. Until all of those are done, the terminal shows an onboarding checklist over the chart and orders will not send.
Crypto is not affected by this.
The contract is not available to your account
Your firm chooses which markets each account can trade. An instrument outside that list will be refused. Expired contracts are also refused, although you can always close an existing position in one.
The order is too large
Two separate caps can stop you. Your firm's contract cap counts open positions plus working orders across every instrument, with micros counted at a fraction of a mini. Crypto carries its own separate position limit in its own units.
Unfilled resting orders are the usual culprit here. Cancel what you are not using and try again.
Orders that reduce your exposure are always allowed, so this never blocks you from closing. An oversized reversal will be caught, though, because it opens a new position on the other side.
One of your own limits stopped it
Personal risk settings are enforced the same way your firm's rules are. A trade cap for the day, a per-symbol contract limit or a blocked symbol will all reject an order. Check the Risk Management page.
Less common causes
No fresh price. Market orders are refused if there is no recent enough price to fill against. Closing and flattening can still fall back to the last known price.
Too many requests at once. There are per-account rate limits on submitting, modifying and cancelling. Rapid-fire clicking can trip them. Wait a moment and retry.
An exit with nothing to exit. A take profit or stop attached to a position that is already flat is refused rather than filled.
If none of these fit
Note the contract, the order type, the size and the time, and contact support from inside the terminal. Your account context comes through automatically.