For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Geckito:SDK:Main/15.5/Geckito:SDK:Main.repo
zypper refresh
zypper install xz-static
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Geckito:SDK:Main/15.4/Geckito:SDK:Main.repo
zypper refresh
zypper install xz-static
For 15.3 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Geckito:SDK:Main/15.3/Geckito:SDK:Main.repo
zypper refresh
zypper install xz-static