Facing issue with after ubuntu setup and before hadoop installations.

Hello,

I have installed ubuntu successfully and now I have to install Hadoop & java. I have created user hadoop for ubuntu and to install hadoop & java first I have to add to sudo but I am not able to do that, not able to edit /etc/sudoers file, not able to work as superuser or admin user.

root@unbuntu:~# usermod -a -G sudo hadoop
usermod: cannot lock /etc/passwd; try again later.

root@unbuntu:~# usermod -a -G admin hadoop
usermod: group 'admin' does not exit.

root@unbuntu:~# ls -l...

Facing issue with after ubuntu setup and before hadoop installations.