В /etc/logrotate.conf
Различаются ли права пользователя в выпусках Ubuntu 20.04 и 18.04?
#use the syslog group by default, since this is the owning group
#of /var/log/syslog.
su root syslog
или
#use the syslog group by default, since this is the owning group
#of /var/log/syslog.
su root adm