Debian installation with encryption: "No root file system is defined"

I'm installing Debian, as I've done a million times, except this time I'm trying with full-disk encryption from the installation itself and partitioning manually, because I'm replacing an existing Linux partition (different distro) on a disk with all sorts of mystical partitions that I don't want to mess with. At any rate, I just need one partition for this installation, whose previous Linux distro I'm replacing with Debian.

However, I seem only able to set the target partition (/dev/nvme0n1p5) as the root partition (/) before I configure its encryption, but then finding myself unable to set it as the root partition once encryption is set up. Therefore, I cannot finish partitioning, because the installer won't let me move on without defining a root partition.

So it's a bit of a catch-22. Configuring the encryption removes the root partition setting, but I can't set an encrypted partition as the root partition.

Not sure if I'm missing something or if this is just some sort of bug in the installer.

I finally figured it out and will leave the solution here in case others have the same problem.

The problem was that the installer expects encryption to be used with the Logical Volume Manager. So I created an LVM group on the partition I was trying to install to (/dev/nvme0n1p5). It then turned out that I actually needed two, one for boot (/boot) and one for root (/). I created a logical volume of 100 MiB for the boot partition, and used the rest for the root partition. (Important: A comment below suggests that 100 MiB for the boot partition is way too low. I've seen some on the internet recommend anything from 300-1000 GiB.)

I was then able to specify those partitions' mount points after having done the LVM stuff and setting encryption on the root partition (the boot partition remains unencrypted), and was then able to proceed with the installation.