"This session is in the past: its students can no longer be notified."

Where: the Notify students sheet (after Cancel session, Restore, or moving a single session in Schedule), when opening the Preview or sending.

Why: the session date is before today (school time zone). Notifying students about a session that has already passed is not offered. A session today, even one that has started, can still be announced.

What to do: nothing to fix for the past session. To write to the students anyway (for example to apologize), use the Messages screen and choose the class's group or groups.