This is a somewhat old problem, since Debian moved to systemD, instead of editing the file in /etc/default/varnish, you will need to create a file in /etc/systemd/system/ named varnish.service, the contents of such a file should look like this, xxx.xxx.xxx.xxx is the IP varnish is listening on, one of the IPs of your varnish server
So to run the following command
systemctl edit varnish.service
[Unit] Description=Varnish HTTP accelerator Documentation=https://www.varnish-cache.org/docs/6.1/ man:varnishd [Service] Type=simple LimitNOFILE=131072 LimitMEMLOCK=82000 ExecStart=/usr/sbin/varnishd -j unix,user=vcache -F -a xxx.xxx.xxx.xxx:80 -T localhost:6082 -f /etc/varnish/default.vcl -S /etc/varnish/secret -s malloc,256m ExecReload=/usr/share/varnish/varnishreload ProtectSystem=full ProtectHome=true PrivateTmp=true PrivateDevices=true [Install] WantedBy=multi-user.target
Once you have added the file execute the following
systemctl daemon-reload systemctl restart varnish