For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:http/openSUSE_Tumbleweed/server:http.repo
zypper refresh
zypper install varnish
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:http/15.6/server:http.repo
zypper refresh
zypper install varnish
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/server:http/15.5/server:http.repo
zypper refresh
zypper install varnish