In order to install Sierra, the partition must be HFS+ (Mac OS Extended). It was possible during Sierra's beta testing (and afterwards if you wanted) to make the volume it's on APFS. But, APFS itself was still considered beta throughout Sierra and was only intended to be used by those who wanted to fully test APFS ahead of the official specification release in High Sierra.
That said, rule numero uno! Backup before proceeding!
As you'll see next, it's really easy to bork your drive with Disk Utility. It always has been as partitioning is a direct modification of the drive layout, but it seems even more so now. This is going to be lengthy (mostly screen shots) because it's the only way to show how difficult this can be to do.
I have to split the response since the forum thinks 12 images is too many for one post.
My preferred way of creating partitions was to do it from another bootable drive. Seems logical since you can then do whatever you want to another physical drive without trying to work around the volume you're currently booted to on the same drive. Nope. Not any more. Here's what happens.
I have two physical drives with High Sierra on. My SSD, and a hard drive (both in a 2010 Mac Pro tower). I booted to the hard drive and went to try dividing the SSD. Notice that the High Sierra volume I'm booted to has the Finder icon over the drive icon.
The SSD does not.
Why does this matter? Because you literally cannot partition the non-Finder marked drive. Not they way you want to, that is. When you first click the Partition button, you get this sheet. Unless you check the box, you'll see it every time.
Adding an APFS shared volume isn't what I want. And besides, you can only choose another APFS type when you do that.
So I click Partition. Notice on the non-Finder tagged APFS drive, you cannot click the +/- buttons.
As seen, I can select any format type I want, but when you do that, it doesn't create a partition, it is going to wipe out the entire drive (next image)! It does this no matter what type of format you choose.