Sometimes root user and its password may work, but
there are distributions as (K)Ubuntu that root user is configured in a
way I don’t know but you can’t use. In fact you don’t know the root
password since a special user is create and you must to use ‘sudo’ to
run programs with root privilegies.

What you have to do to solve this situation is to edit
/etc/cups/cups.conf with sudo and locate the propieties User and Group.
If you don’t do nothing they might be commented.

Uncomment them and change the user and group to an user/group you have
at the system and you know its password. For example, your user.

User manou
Group manou

I don’t know if this is also needed but I have edited /etc/group file too and I have added this user to lp group.

Then, restart cups daemon and the next time you want to access to cups
manager through http you will be able to enter with your user and
password.

Another thing you can change at /etc/cups/cups.conf file is the access allowed. By default only local access is allowed.

Deny From  All
Allow From 127.0.0.1

change Allow value for 192.168.255.255 to allow all your local network to access to cups.