Some Darknet markets are available by invitation only, which means that new buyers and sellers cannot register unless they have an invitation link from an existing user.
Further, DarknetMarkets.org is constantly updating invitations to all guest-only markets. Follow the links to register new accounts.
If you use these links and the market still says that the records are closed, you will have to wait until the records are opened again. Keep trying, as some markets open and close records regularly depending on the load on the server.