You must go to pgAdmin, in the list on the left look for Login Roles/Groups; Right click and create a new role. The username and password that you must enter is the one that appears in the odoo.conf file that must be inside the Odoo "server" folder. Also in "privileges" check all the options, so you can create your database.

Finally, restart your postgreSQL server.

I hope it helps!

Yo lo resolví, configurando en el server de Postgres el usuario, la contraseña y el puerto que está en el archivo "odoo.conf". 
Yo tenía el server de Postgres en otro puerto y no me funcionaba cambiar el puerto en el archivo, así que tuve que cambiar el puerto del server y además crear el usuario que estaba en el odoo.conf y ponerle esa contraseña que decía ahi en ese archivo.

Hi,

Thanks for your response Manish, and I hope I'm not to rude... but honestly, I don't get how this answer helps someone that doesn't know anything about programming. I have the same problem as Alejandro, and I don't have a clue about what you're saying.

I'm trying to either create a new db, or restore a copy I downloaded from Odoo Online (I need both). And the only thing I do is fill out the creation (or restore) form, where it asks the Master Password, Database Name, email, etc...

And in both cases I get the same error:

Database creation: operational error ()

So I don't know how this links with the Odoo config file (honestly, I don't even know where it is or how to open it).

Are there any step-by-step instructions we can follow to solve this?