For openSUSE Tumbleweed run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Utilities/openSUSE_Tumbleweed/X11:Utilities.repo
zypper refresh
zypper install xsel
For openSUSE Factory ARM run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Utilities/openSUSE_Factory_ARM/X11:Utilities.repo
zypper refresh
zypper install xsel
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Utilities/15.6/X11:Utilities.repo
zypper refresh
zypper install xsel
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/X11:Utilities/15.5/X11:Utilities.repo
zypper refresh
zypper install xsel