To sync quantity to Rakuten, the following values are required:
Rakuten requires the following product data when attempting to sync inventory: Rakuten SKU, Is Offer Expedited Shipping? , Condition, Storefront Price. If any of these values are missing, the sync will fail with a warning message.
To set this values navigate to Inventory - Inventory management and click into a product details using the Binoculars icon. From here, choose the listing tab and click 'listing attributes'. Choose Rakuten and you will see the above values to populate.