Locals often buy train tickets only a month before their journey. Ticket sales start 2-6 months prior to travel, depending on the route and operator. This works well for locals, but foreign travelers may try to book too early, which could lead to unsuccessful reservations. If you attempt to book a ticket too early, you may not see any trains or prices, or you might only see a portion of them, or the booking attempt may result in an error message. Most likely, the booking is not yet fully open.
The time limit of two, three, four, or six months is not exact, and it may be shortened due to schedule changes in June or December. If trains are missing on certain days during the booking period, it may be due to track work. ‘Ticket not available’ or ‘Sold out’ sometimes really means just that. However, if it concerns a reservation that is several months away, it is likely that the ticket is simply not yet on sale.
Companies and countries have their own operational models and timelines for ticket sales. A good general overview can be found at: https://www.seat61.com/european-train-booking-horizons.htm
On discussion forums, you can see posts where someone planning a trip asks if they are too late to book a ticket for their journey several months away. It is possible, but it’s likely they may be trying to book too early.