In my limited experience with APFS, what you're seeing is a result of no partition map existing (yet) on your Mac. APFS's metaphor utilizes Volumes - dynamically resizable containers within the Mac's Container. Read my recent
post here for my input/responses in a thread about a user wanting to install Mojave alongside HS, along with a post two posts below (with an external link with more info about Volumes...), and my additional post later in that thread. I have 3 Volumes on my iMP - HS, Mojave, and Windows 10. I no longer use partitions as Volumes serve exactly the same purpose but with additional benefits and with no worries about partition map corruption and reformatting. Partitioning is IMHO a legacy of the 90s that needed to be put down.
If you truly want to go with partitions, you'll need to boot into Recovery Mode and use Disk Utility in that Mode - Disk Utility will then be able to create a partition map.
