Could not lock session directory: "/home/user/rtorrent/.session/" held by user
It usually results from not closing rtorrent properly, or due to abrupt ending of an rtorrent session.
To resolve this, delete the rtorrent.lock file,
Find rtorrent.lock file in rtorrent session directory, to get the location of session directory, see rtorrent config file .rtorrent.rc
remove rtorrent.lock file using rm