If you see the following error:
We're unable to complete your request because this SKU is not in the Amazon catalog. If this was a deleted SKU, wait 24 hours before resubmitting it. If you tried to add this SKU to the Amazon catalog before, check your data and correct any errors before resubmitting.
It means that the account has used the SKU previously for a listing. You must use a unique, unused SKU upon creation.