Error (1): The specified org [my_username] is not a Dev Hub.

Badge: Quick Start: Explore the Easy Spaces Sample App

Step: Explore the Easy Spaces App Packaging Architecture

I already enabled Dev Hub on the playground org.

Set the Trailhead Playground as the default Dev Hub using the following command:

sfdx config:set defaultdevhubusername=[my_username]

That seems to have been successful:

Name                                    Value                            Success

─────────                     ────────────    ─────────────

defaultdevhubusername  [my_username]           true

Now I try

sfdx package:create --name=ESObjects --packagetype=Unlocked --path=./es-base-objects     

and get this error:

Error (1): The specified org [my_username] is not a Dev Hub.

Could somebody tell me what am I doing wrong?