Technically speaking, Office in the Windows Store is in "preview" -- a Microsoft support article states that "while Office itself is not in preview, Office is using new install and update processes that are in preview." Because of that early release status, Microsoft is giving a free one-year subscription to Office 365 personal to with a Windows 10S device. If you're not using a Windows 10S device, you won't be able to install Office via the Windows Store until it's out of preview (fortunately, there are plenty of other ways to get Office on those computers).

Microsoft notes a few other restrictions for Office on Windows 10S: it's only a 32-bit version, the COM add-in tools won't work and the OneNote 2016 desktop app isn't ready yet. You'll have to install the universal Windows Platform app instead. For most people, these won't be deal-breakers, and the free year of access should more than make up for any pain here.