The official instructions worked for me, except of that they don’t mention the --stable flag. Without this, it cannot find an image. But the --stable versions seems to be fully encrypted by default,
./scripts/librem5-flash-image --help
says
--variant {plain,luks}
Image variant to download ( plain, luks ), default is 'luks'
Besides this, there seem to be newer versions of pureOS, than the one provided b y the script: Latest release and version history overview