By default the permissions on the
.sessions directory is 777. Additionally, you shouldn't have to add
service before a systemctl command.
I would not suggest anyone look to this as a fix for issues they may have starting rtorrent as you are showing that removing the rtorrent directory and adding it back is a plausible solution and that is a very problematic/destructive for people with data in that directory.
Just make sure the permissions are set and the directory is owned by your user.