I can help you in fixing your error.
You need to restart your pc first, and after that keep pressing the ESC key while your Ubuntu is booting.
This procedure will bring up your boot menu for you.
Now you need to select the Advanced Options.
After that carefully select your OS version in (recovery mode), and after that just press Enter Key.
It should bring up another screen for you. Now just select “Drop to root shell prompt” and after that press the Enter key.
Now you will the command line loaded at the bottom of the screen.
Now the most important part begins here you need to run very carefully each of the below commands.
mount -o remount,rw /
chown root:root /usr/bin/sudo
chmod 4755 /usr/bin/sudo
shutdown -r now
Now your error must be fixed.