The varnishcache/varnish60lts repository (long-term support packages for Varnish 6.x) on packagecloud.io does not provide a package for Debian 10: github.com/varnishcache/pkg-varnish-cache/issues/127#issuecomment-608462425
The official Debian repository provides only an outdated 6.1.1 version of Varnish: packages.debian.org/buster/varnish
This version of Varnish was released at 2018-10-26: github.com/varnishcache/varnish-cache/releases/tag/varnish-6.1.1 .
It can be installed via the standard Debian way:
aptitude install varnish