No Plasma Mobile on PureOS right now, sorry - Debian Buster (which is what PureOS Amber is based on) is simply a too old base to reasonably support Plasma Mobile.
I do hope that we’ll be able to offer Plasma Mobile with the switch to Byzantium though (the next release based on Debian Bullseye). Meanwhile, you can use other distros that already include PlaMo on your Librem 5, such as postmarketOS.