Using Sudo Gives Me the Error: Sorry, Try Again

This is what happens when I try :

sudo gedit

[sudo] password for casper: 
Sorry, try again.

My username is casper.

I seem to be in the sudo group :

id | grep sudo
uid=1000(casper) gid=1000(casper) 
groups=1000(casper),4(adm),24(cdrom),27(sudo),30(dip),46(plugdev),116(lpadmin),12 6(sambashare)

Auth log looks like this :

tail /var/log/auth.log
May 16 18:39:18 casper-pc pkexec: pam_unix(polkit-1:session): session opened 
for user root by (uid=1000)
May 16 18:39:18 casper-pc pkexec[2426]: casper: Executing command [USER=root] 
[TTY=unknown] [CWD=/home/casper] [COMMAND=/usr/lib/update-notifier/package- 
system-locked]
May 16 18:42:16 casper-pc pkexec: pam_unix(polkit-1:session): session opened 
for user root by (uid=1000)
May 16 18:42:16 casper-pc pkexec[5463]: casper: Executing command [USER=root] 
[TTY=unknown] [CWD=/home/casper] [COMMAND=/usr/lib/update-notifier/package- 
system-locked]
May 16 18:45:16 casper-pc pkexec: pam_unix(polkit-1:session): session opened 
for user root by (uid=1000)
May 16 18:45:16 casper-pc pkexec[7356]: casper: Executing command [USER=root] 
[TTY=unknown] [CWD=/home/casper] [COMMAND=/usr/lib/update-notifier/package- 
system-locked]
May 16 18:46:55 casper-pc sudo: pam_unix(sudo:auth): authentication failure; 
logname= uid=1000 euid=0 tty=/dev/pts/0 ruser=casper rhost=  user=casper
May 16 18:47:00 casper-pc sudo: pam_unix(sudo:auth): conversation failed
May 16 18:47:00 casper-pc sudo: pam_unix(sudo:auth): auth could not identify 
password for [casper]
May 16 18:47:00 casper-pc sudo:   casper : 1 incorrect password attempt ; 
TTY=pts/0 ; PWD=/home/casper ; USER=root ; COMMAND=/usr/bin/gedit

Could someone please help me restore sudo? Thank you in advance.