This error message occurs because PeopleHR Planner only has the ability to book or edit a leave for the current, previous, and following holiday year.
Although the old leaves are already booked, any change applied on the leave will be considered as an update, and thus can't be saved to the system.
📌Note: If your holiday year is Jan - Dec, if you try to add an entry for 2027 this is too far as it can only look at the current, previous, and following holiday year.