Shopify Product Creation Rate Limit
Once a Shopify store has 500,000 variants, Shopify will allow no more than 10,000 new variants to be created per day. This limit does not apply to stores on the Shopify Plus plan.
Each product you push from Spark Shipping creates one variant in Shopify, so pushing a large catalog to a store that already has 50,000 variants can take many days. Push in smaller batches, see Best Practices for Item Catalogs.