Hello @elisa123456 

Find the auth.json file in the Magento root directory.

If it does not exist, create the file and enter public and private key as shown here:

{
  "http-basic": {
    "repo.magento.com": {
      "username": "enter-public-key",
      "password": "enter-private-key"
    }
  }
}

Make sure that you have valid authenticate keys.

You may refer https://devdocs.magento.com/guides/v2.3/install-gde/prereq/connect-auth.html

Thanks.