This is a postgres authentication error. You're trying to run OpenERP as root (not recommended), but your postgres server is not allowing local peer authentication on root.

I suggest making a system account named 'openerp':

sudo adduser --system --home=/opt/openerp --group openerp

Use this account to start and stop the server. You'll need to chown the entire openerp folder, and I don't recall offhand where the .deb package puts that. Once that's done, check pg_hba.conf in your postgres configuration folder and make sure local connections allow 'peer' authentication on all databases for the 'openerp' user. The comments in that file explain how to set that up properly. IIRC it looks something like this:

local    all    openerp        peer

Postgres will check the system login making the request to connect, and if it's the 'openerp' user, immediately grant database access.