For openSUSE Factory run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Archiving/openSUSE_Factory/Archiving.repo
zypper refresh
zypper install stuffit
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Archiving/15.6/Archiving.repo
zypper refresh
zypper install stuffit
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/Archiving/15.5/Archiving.repo
zypper refresh
zypper install stuffit