How to set node id as SKU id
In Drupal Commerce products are entities, not nodes. If you're looking for a product's SKU to be automatically set to the created product's id, there's the Commerce AutoSKU module which allows you to use tokens (e.g. [product:product-id]) to automatically create SKUs.

One caveat from the project however:


  If you decide to use the [product:product-id] token, you should be
  aware that this will cause any new products to be saved twice. This
  shouldn't be an issue for most people, but if you have rules in place
  for when a product is saved, you may not want to use this token.