I don’t know LMDE, but apt-get needs entries in /etc/apt/sources.list. They define repositories storing packages.
On my Ubuntu, there’s deb https://ubuntu.ethz.ch/ubuntu/ mantic main restricted
This specifies one repository to find ‘.deb’ files.
deb-src is another possible prefix, to specify repositories storing source code. An example from Ubuntu is: deb-src http://ubuntu.ethz.ch/ubuntu/ mantic-security restricted main multiverse universe
It seems a deb-src repo is needed by the command you issued, and you do not have any.