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