1. First of all you have to check admin_passwd in odoo.conf. But if you see something like

admin_passwd = $pbkdf2-sha512$25000$15oTopRSSomREkLIeQ8hRA$QlTTxd4bqTTxSI0/u4g/yukO9jT2yFtTx4sAu46GtMKHnooPWT 

Then you can't know what is db-admin password.

2. But you can change it like this:

admin_password = my-New-Cool-P@$$WoRD

Restart odoo server by command: "sudo service odoo restart" and try to create DB again.

3. But if it not worked! Maybe your parameters db_user or\and db_password is not correct. Check it in /etc/odoo/odoo.conf

If you use postgresql with the specified user and password,
then you must specify the same user and password in /etc/odoo/odoo.conf in parameters db_user and db_password.
Otherwise, you will get this same error due to the fact that the database user has no rights to operations with databases.

For example, if you use the official docker, and docker-compose from the site https://hub.docker.com/_/odoo/ or https://store.docker.com/community/images/library/odoo then pay attention to the parameters

  mydb:
    image: postgres:10
    environment:
      - POSTGRES_USER=database-user
      - POSTGRES_PASSWORD=my-databaseuser-password
      - POSTGRES_DB=postgres

Thus, in order for you to do something with the databases from https://yourdomain/web/database/manager, you must set the db_user=database-user and db_password=my-databaseuser-password parameters in the file /etc/odoo/odoo.conf

4. If you don't know what is db user name and db-user password, then try to set official db_user=odoo and db_password=odoo.
5. If it not helped then you can try to  add new postgresql user, give him all permission and use him in odoo.conf
6. Or you can change postgresql user password.