For 16.0 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:andreas.osterburg:Radicale/16.0/home:andreas.osterburg:Radicale.repo
zypper refresh
zypper install Radicale
For 15.6 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:andreas.osterburg:Radicale/15.6/home:andreas.osterburg:Radicale.repo
zypper refresh
zypper install Radicale
For 15.5 run the following as root:
zypper addrepo https://download.opensuse.org/repositories/home:andreas.osterburg:Radicale/15.5/home:andreas.osterburg:Radicale.repo
zypper refresh
zypper install Radicale