If you can not save your new created extension to a correct path then maybe a additional first entry in your project composer.json is needed.

"repositories": 
  [ 
    { "type": "path", "url": "./packages" },
    ...
    ...
  ]

There will be now a „root“ storagePath an the extension could be saved in the correct location. In my case without this config it would be accidently saved nested into the folder of another extension.

TYPO3 Version
10.4.23 composer mode

Extension Builder Version:
git v10.0.3