The account you're writing from has no custom domains added to any service, and its only service, a Postgres database, has no active deployment, so there is nothing for that domain to route to yet.

To serve a domain, open the Settings of the service that runs your app, click + Custom Domain in the Public Networking section, and add both the CNAME and TXT records it shows at your DNS provider, as described in our domains guide. The domain returns this Not Found page until both records are in place and verified.

If the domain is attached to a project under a different account, it has to be managed from that account.