You need to install PureOS from the beginning correctly, repeat everything from the USB Flash Drive (with the PureOS image on it). (I’m going out now, just please stay focused on what written within related install guide and you’ll be fine).
I see that you do not have there /dev/sdb1 option (mounted USB Flash Drive = with PureOS install image on it). Please power off your Librem 13v4, connect power supply to it, connect USB Flash Drive to USB-A port, turn on your Linux laptop and let us know if /dev/sdb1 present/visible?
EDIT: @rcjav, hello, is this shortcut the one that you are looking for? Chapter containing this part: “which mean /dev/sda1, /dev/sda2, and so on” is the one that you might need there, pointing toward there used USB Flash Drive.
As another option/solution, you might want to take this Librem 13v4 solid-state drive out and replace it with the another one. Or rather take this current (yet wrongly partitioned) SSD out (put it in some external SSD enclosure) and erase/reformat currently encrypted SSD to contain only one simple partition and, bring it back inside of Librem13v4 and start brand new PureOS install afterwards.
@Quarnero I am very dependent from my laptop in a daily basis, so I bought a new SSD and PureOS is now running again. Problem SOLVED!
I am so grateful for the advice, useful links (my knowledge will boost) and time spending around this issue. Many thanks!