As a Canvas Catalog admin, you can enable SKU uploads in the Catalog Info page. By default, SKU uploads are not allowed. However, you can edit a catalog or subscatalog's Info page to allow SKUs to be added manually to individual listings or bulk uploaded via API.
Note: Canvas Catalog now supports SKUs for many payment providers, such as Cashnet, Common Checkout Page (CCP), Paypal, Touchnet, etc.