Hi,
I saw you are using Dawn theme and products have SKU. However, code above is not show.
I think this code used " product" instead “card_product”. You try code below:
{{ product.selected_or_first_available_variant.sku }}
If it doesn’t work. You can share code of this file Go to Store Online-> theme → edit code → Snippets/card-product.liquid ? Or let me access to your store to check it.