if you want to user pure-ftpd on backtrack or if you already installed and you get an error, just fallow along and you will never see the error again
install pure ftp daemon:

apt-get install pure-ftpd

we will add a user to the linux because we do not want the ftp get a root access

atftpd --daemon --port 69 /tmp
groupadd ftpgroup
useradd -g ftpgroup -d /var/www/ftp -s /etc ftpuser
(chose a password)
pure-pw useradd ftpuser -u ftpuser -g ftpgroup -d /var/www/ftp
chown -R ftpuser:ftpgroup /var/www/ftp
ln -s /etc/pure-ftpd/conf/PureDB /etc/pure-ftpd/auth/60pdb
echo no > /etc/pure-ftpd/conf/PAMAuthentication
echo yes > /etc/pure-ftpd/conf/DontResolve
pure-pw mkdb
pure-ftpd start
pure-ftpd restart

do: nano /etc/shells
and replace everything with(adding just the last two lines):

# /etc/shells: valid login shells
/bin/csh
/bin/sh
/usr/bin/es
/usr/bin/ksh
/bin/ksh
/usr/bin/rc
/usr/bin/tcsh
/bin/tcsh
/usr/bin/esh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen
/bin/false
/usr/sbin/nologin

reboot
now your ftp should work just fine!