The /etc/apt/sources.list file has no "deb-src" entries after the OS installation, so "apt source" can't find package sources.
The following lines should be added to the sources.list:
deb-src https://repo.pureos.net/pureos/ amber main
deb-src https://repo.pureos.net/pureos/ amber-security main
deb-src https://repo.pureos.net/pureos/ amber-updates main