I'm not an expert on this matter neither but I think that you completed:
So you already have a new LVM logical volume (LV), don't you?. Anyway, read https://www.digitalocean.com/community/tutorials/an-introduction-to-lvm-concepts-terminology-and-operations just to be sure you understand the...
I had the same problem after installing PVE on top of a regular Debian (i.e. without using the PVE ISO Image), so I had to do something like this to be able to create templates:
1. Reduce the size of my root partition (this was providing the Directory Storage).
2. Use the newly freed space to...
Here you have another step by step guide, https ://gist.github.com/Yub0/518097e1a9d179dba19a787b462f7dd2#gistcomment-2625690.
Note that it is important to use Proxmox's pct to stop and start containers instead of lxc's tools.