pkg: No packages available to install matching 'sysutils/rdiff-backup' have been found in the repositories
pkg installdoesn't take the category prefixes like "www/" so it's supposed to be
pkg install rdiff-backup.
pkg search rdiff, to name one example.
pkg: No packages available to install matching 'rdiff-backup' have been found in the repositories
colordiff-1.0.19 Tool to colorize diff output dirdiff-2.1_4 Graphical diff tool py37-rdiff-backup-2.0.0 Easy to use and efficient backup and restore utility
pkg install py37-rdiff-backup, drop the version number when installing.