For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Sauerland:sane/openSUSE_Tumbleweed/home:Sauerland:sane.repo
zypper refresh
zypper install hplip
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Sauerland:sane/15.5/home:Sauerland:sane.repo
zypper refresh
zypper install hplip
For 15.4 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:Sauerland:sane/15.4/home:Sauerland:sane.repo
zypper refresh
zypper install hplip