You need to set up fulfillment service if you use Dropwow APP together with another dropshipping automation service like Oberlo.
Update Dropwow App in Shopify store. Go to Settings in Shopify dashboard. Scroll down and check "Active dropshipping and fullfilment services".
To choose the fullfilment service for a certain product you need to set it up for every SKU of the product. Open the product you need. Click "Edit" and choose "Dropwow fulfillment service". Save the changes.
You can also set up fulfillment service for all variants of the product at once. Select all variants and click "Edit options". Add the field "Fulfillment service". Select Dropwow. Save the changes.