I have found the solution here: github.com/varnishcache/pkg-varnish-cache/issues/127#issuecomment-607898105
Step 1
I removed an outdated (6.1.1) Varnish version provided by the official Debian repository:
aptitude remove varnish
Step 2
aptitude update
aptitude install debian-archive-keyring
aptitude install curl gnupg apt-transport-https
curl -L https://packagecloud.io/varnishcache/varnish64/gpgkey | sudo apt-key add -
packagecloud.io/varnishcache/varnish64/install#manual-deb
Step 3
I added to /etc/apt/sources.list:
deb https://packagecloud.io/varnishcache/varnish64/debian/ buster main
deb-src https://packagecloud.io/varnishcache/varnish64/debian/ buster main
packagecloud.io/varnishcache/varnish64/install#manual-deb
Step 4
aptitude update
aptitude install varnish=6.4.0-1~buster
packagecloud.io/varnishcache/varnish64/packages/debian/buster/varnish_6.4.0-1~buster_amd64.deb
Step 5
Checking the result:
varnishd -V
