I just installed commerce on Drupal 8. I created a Store, Product Type, and a few products. When I was creating the products I was often getting an error message that the sku was already in use. If I create a view based on the Product Variations and show "sku" and "Product Variation Title" I the result shows that there are skus that are not associated with a Product Variation Title. In these cases there are multiple listings of the same sku with different UUID and IDs. I am manually entering the sku on the product add form. The unassociated skus are sequential to the sku I did enter when I created products. What did I do wrong?
Install the Commerce Bulk module and create or change your SKUs in bulk. Also, I'd recommend to check if you have any orphaned variations (no parent product) which are legal in Drupal Commerce 2.x but has no any UI to manage them. If you are sure that those should not exist on your site then just delete them all: https://www.drupal.org/project/commerce_bulk/issues/3027034#comment-12933162