Certified GKH Gardening Expert
It appears to be insect damage, lead to fungal infection. It would be hard to say if it were a moth, or some other insect, but it was definitely an insect.
The best treatment from here will be an insecticide. This article will help: https://www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm
This article will offer more information on growing cabbage: https://www.gardeningknowhow.com/edible/vegetables/cabbage/growing-cabbage.htm