Why can't I delete some notes

[ChipAHoy 7]

I am doing a cleaning of my saved information in EN, and selecting several notes at once for deletion. Sometimes the delete button is greyed out. Why does this happen?

Also, I tried specifically deleting some notes labeled as EverMetaNotes, but these will not delete, either.

I have been able to delete other blocks of notes successfully.

Using Mac Evernote for Desktop.