That room is blocked for those nights
Somebody blocked the room from the chart. The refusal says the dates, the purpose and who. Open the block or pick another room.
"Room 205 is blocked 20 Sep to 25 Sep — Renovation." The room was blocked from the chart (or closed on the Rooms screen) across at least one of the nights you want, and a booking into it is refused by the database — not just hidden by the chart, so a booking from any screen meets the same answer.
- Pick another room, or
- right-click the blocked night → Open the room, if the block is over. The channel is told the room is for sale again.
"Room 205 is already blocked … open it first" is the other direction: you are blocking a room that carries a block. One at a time — open the first.
The Blocked rooms log (right-click any room) says who asked for each block, who entered it, and who opened it.