For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:apps/openSUSE_Tumbleweed/home:darix:apps.repo
zypper refresh
zypper install rubygem-mixlib-config
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:apps/15.6/home:darix:apps.repo
zypper refresh
zypper install rubygem-mixlib-config
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:darix:apps/15.5/home:darix:apps.repo
zypper refresh
zypper install rubygem-mixlib-config