The Rakuten SKU can be found on Rakuten when you click on a listing. If you scroll down, you will see a specifications section. (See screen shot attached)
This is the SKU number ecomdash makes a match on for listing and syncing to Rakuten.
The Rakuten SKU is displayed on a Rakuten Listing as the "Base SKU"
The Rakuten SKU can also be found in the listing URL:
Rakuten SKU= 285721973