The period is released
A note above the grid says so, and every cell is locked. The card a learner already holds must not change underneath them.
Fix: Class → Release cards, then Take back the release, give a reason if it helps you remember, and confirm with Take it back. Mark what you need, then release again — the new cards carry a revision number.
The learner is not active
A learner marked transferred out or dropped keeps their status pill next to their name and their cells are greyed. Marks they already have stay on the card and still print.
The class is archived
An archived class is read-only everywhere, so the grid shows the marks but takes none, and Manage core values is gone. Fix: Class settings → Reopen in the archived note.
You are a subject teacher, not the adviser
If you teach a subject in someone else's section, Conduct opens read-only. Observed values are the adviser's report, and they are the only one who can mark them.
The core value has just been removed
If the value was removed in another tab or on another device, the mark is refused with "That core value is not on this class's list any more. Reload the page to see the current list." Reload and the grid shows the list as it stands.
If none of these fit
The two other refusals name themselves plainly: "That learner is not on this class roster" and "That grading period does not exist for this class". Both mean the page is showing something older than the class is; reload it. If a cell still refuses a mark after a reload, message us on the Facebook page during business hours with your support reference and the class code.