The order button is greyed out or you see "Market closed".
The instrument is outside its trading hours, or it is the weekend.
Every symbol carries its own trading session times, set by the broker and stored in the symbol specification. The terminal reads those times and disables order entry outside them, which is why the platform can be fully connected and streaming while one instrument refuses to trade. Sessions are defined in the broker's server time, not yours, so an instrument can look closed at a time that seems wrong until you account for the offset between your clock and theirs.
Brokers also set different sessions for different instrument classes on the same account. Forex generally runs continuously from Sunday evening to Friday evening with a short daily break, index and commodity CFDs follow their underlying exchange hours with their own breaks, and share CFDs follow the exchange calendar including public holidays. A holiday closure on one exchange will therefore close some symbols and leave others trading normally.
Right-click the symbol in Market Watch and open its specification, then read the trading session times listed there and compare them with the server clock shown in Market Watch rather than your own computer clock. When the session opens, the bid and ask start moving and the New Order dialog will accept a market order for that symbol.
Check the session times in the symbol specification before you plan a trade on an unfamiliar instrument, and note that they are in server time. Ask your broker for their holiday schedule if you trade indices or shares, since those closures are announced by the broker and are not visible in the platform in advance.
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