Specified Username Already Exists or Is Reserved.

What username are you trying to create?  There are quite a few built-in accounts

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/bin/sh
bin:x:2:2:bin:/bin:/bin/sh
sys:x:3:3:sys:/dev:/bin/sh
sync:x:4:98:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/bin/sh
man:x:6:12:man:/var/cache/man:/bin/sh
lp:x:7:7:lp:/var/spool/lpd:/bin/sh
mail:x:8:8:mail:/var/mail:/bin/sh
news:x:9:9:news:/var/spool/news:/bin/sh
uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh
proxy:x:13:13:proxy:/bin:/bin/sh
www-data:x:33:33:www-data:/var/www:/bin/sh
backup:x:34:34:backup:/var/backups:/bin/sh
list:x:38:38:Mailing List Manager:/var/list:/bin/sh
irc:x:39:39:ircd:/var/run/ircd:/bin/sh
gnats:x:41:41:Gnats Bug-Reporting System (admin):/var/lib/gnats:/bin/sh
media:x:71:99::/var/cache/forked-daapd:/bin/false
statd:x:79:99::/var/lib/nfs:/bin/false
libuuid:x:80:80::/var/lib/libuuid:/bin/sh
sshd:x:82:99::/var/run/sshd:/usr/sbin/nologin
messagebus:x:83:83::/var/run/dbus:/bin/false
avahi:x:84:84:Avahi mDNS daemon:/var/run/avahi-daemon:/bin/false
nut:x:85:85::/var/lib/nut:/bin/false
snmp:x:86:86::/var/lib/snmp:/bin/false
klog:x:89:89::/home/klog:/bin/false
syslog:x:91:91::/home/syslog:/bin/false
ReadyNAS:x:96:96::/var/lib/timemachine:/bin/false
ftp:x:97:97::/var/ftp:/bin/false
admin:x:98:98::/home/admin:/bin/false
guest:x:99:99:nobody:/nonexistent:/bin/false
nobody:x:99:99:nobody:/nonexistent:/bin/false