Steps to reproduce in net-installer, more or less quoted from
https://www.reddit.com/r/debian/comments/146vw37/guide_install_debian_with_encrypted_root_and/
Installer menu Partitioning
1 Select "Manual Partitioning"
2 Select the free space/partition and choose "create partition."
3 Use As: Physical Volume for Encryption
4 Scroll up to "configure encrypted volumes" > write changes, then
"create encrypted volumes."
5 Select "partition" > "finish"
6 Create your encryption password
7 Partition disks....
8 Scroll to "encrypted volume" and select the partition listed beneath
to edit it.
9 Set mount point to "/" (root) > done setting up partition
10 Finish partitioning and write changes to disk
Typically net-installer does not offer step 8 while DVD-installer does.
In other words, "encrypted volume" is simply not visible.
As a consequence it is impossible to do step 9: select the volume and
assign a mount point.
Summarized, installing Debian using the net-installer on an encrypted
root partition is impossible.
Best regards, Wiebe