Installation of PostgreSQL-11: Depends: libllvm6.0 (>= 1:6.0~svn298832-1~) but it is not installable
This error is because the Official PostgreSQL PPA for Cosmic is currently building against libllvm6.0. That's an import from Debian Sid, currently Ubuntu is building against the newer libllvm7.0. To get access to this package you'll have to add the universe to your sources.list.

sudo add-apt-repository "deb http://archive.ubuntu.com/ubuntu $(lsb_release -sc) universe"