"This session no longer exists: the class may have been changed or moved to the trash."

Where: opening a session (Today, Attendance, Schedule) or Finish.

Why: the session no longer exists on that date: the class was moved to the trash, its repetition changed ("This and following sessions"), or the one-off session was deleted.

What to do: go back to Schedule or Today to see the current sessions. A class or one-off session in the trash can be restored for 30 days (Settings > Data > Trash, owner).