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