Please either use environment variables or create a .env file in the backend root directory (in which this readme file is located) to specify the DATABASE_URI.
Example contents of a .env file for the backend:
DATABASE_URI="postgresql://<username>:<password>@<host>/<database_name>"
BACKEND_CORS_ORIGINS=["http://localhost", "http://localhost:4200", "http://localhost:3000", "http://localhost:8080", "https://localhost", "https://localhost:4200", "https://localhost:3000", "https://localhost:8080", "http://dev.oasst.laion.ai", "https://stag.oasst.laion.ai", "https://oasst.laion.ai"]
Have a look into the main README.md file for more information on how to set up the backend for development.