Why is BUY/SELL greyed out or 'market closed'?

Market orders are only accepted while the instrument's market is open, so the terminal disables BUY/SELL and shows 'market closed' outside those hours. Crypto trades 24/7. Forex and metals trade around the clock on weekdays but close over the weekend (roughly Friday evening to Sunday evening, Central European Time). Cash indices trade about 07:00–22:00 CET on weekdays, and US stocks and ETFs about 15:30–22:00 CET on weekdays; all of these are closed on weekends. You can see a symbol's exact session hours in its Symbol Specification (Sessions tab). If a market is closed, wait for it to reopen, or place a pending order that will trigger once trading resumes.