It seems that Odoo is unnable to connect to postgreSQL, this may have several reasons:

• Is postgres working and running? If you use windows press Windows Key + R, type "services.msc" and check if service called PostgreSQL_For_Odoo is running
• If Odoo was working and you did something and started getting this error, than try to check if what you did caused Odoo to stop communicating with postgre. That may be the case if you edited odoo.conf file and removed the credencials
• You may also want to check if the user "db_user" on odoo.conf file exists on the postgreSQL