You see open nights between two existing reservations, but guests cannot book them.
Why this happens. The website enforces the stay rules set in your PMS, so a window of open nights becomes unreachable when no valid stay fits inside it. There are two usual causes: the gap is shorter than the minimum-stay rule for those dates, or the only possible arrival day in the window is closed to check-ins in your PMS. Turnover and arrival-day restrictions do this.
What you can do. To make the window sellable, adjust the rule in your PMS. Lower the minimum stay for those dates, or open the first day of the window to arrivals. The website picks up the change automatically on the next sync. Nothing needs to change on the site itself.