How to access and create a new PostgreSQL user in Ubuntu?

Question

PostgreSQL

Performed a standard installation

sudo apt update
sudo apt install postgresql postgresql-contrib

I'm trying to create a new user
sudo -u postgres createuser <username>
But nothing happens
To access the command line I write
sudo -u postgres psql
AND I get - postgres is not in the sudoers file. This action will be logged.
I create a new scheme
createdb mydb
Everything works here, I can add and change entries.