For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_devel_languages_nodejs/openSUSE_Tumbleweed/home:ojkastl_buildservice:Branch_devel_languages_nodejs.repo
zypper refresh
zypper install pnpm
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:ojkastl_buildservice:Branch_devel_languages_nodejs/15.6/home:ojkastl_buildservice:Branch_devel_languages_nodejs.repo
zypper refresh
zypper install pnpm