How to fix "SKU can't be blank"?
By default, BYOB generates new variants to support bundle discounts. And Shopify doesn't require an SKU when creating new products/variants.
Therefore, it might be a recent issue of Shopify: https://community.shopify.com/c/shopify-discussions/sku-can-t-be-blank-error-when-sku-is-not-blank/td-p/1931945
You can activate the option - Add SKUs to dummy variants under the BYOB settings => Fulfillment. When there is an SKU added by BYOB, the error will be fixed