An FBA offer already exists on this SKU
An FBA offer already exists on this SKU. To create a seller fulfilled SKU, either create a new SKU or make the SwitchFulfillmentTo value MFN.
When Salestio sends a Shopify product to Amazon, it sets the Fulfillment Channel as MFN. If a product with the Shopify product SKU exists with an FBA fulfillment channel in your Seller Central account, Amazon returns the error like this.
If you would like to set both fulfillment methods for your products, you should create different SKUs for each fulfillment.