You are not providing any dates. Without dates Shopify cannot calculate the business days to delivery. Hence you are not returning enough information. For testing purposes, you will need to try a LOT of dates to satisfy your needs. Shopify has all sorts of undocumented quirks when it comes to processing your dates. Things like weekends. Make sure you provide good minimum and maximum dates and all will likely work.
I fought this API for some time, and getting it in shape was not easy...