Reckon One doesn't have an inventory feature currently. It is on the road map and will be available in the future. I don't have an ETA on this however.
Reckon Accounts Desktop and Reckon Accounts Hosted have an extensive inventory feature, plus the desktop version can integrate with a third party add on called Fishbowl that has even more extensive inventory features. I'm not sure if Fishbowl are working on integrating with Reckon One but I will check and get back to you.