Para 15.6 corra o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.6/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-core
Para 15.5 corra o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.5/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-core
Para 15.4 corra o seguinte como root:
zypper addrepo https://download.opensuse.org/repositories/home:DarkWav:mono-backports/15.4/home:DarkWav:mono-backports.repo
zypper refresh
zypper install mono-core