First, verify that the instance you’re connecting to is the correct one - either via their landing page or via the Securedrop directory at https://securedrop.org/directory/ . It is possible (though very unlikely) that a malicious actor could set up an onion service masquerading as a legitimate SecureDrop instance. If the instance uses HTTPS, you can check that as well using the icon on the left-hand side of the address bar.
If you are confident that you have the right address, visit it using the latest Tor Browser version, with the Security Level set to Safest (via the shield icon on the right-hand side of the address bar), and try creating a new code name by choosing Get Started, and then Submit Documents.
If this works, then the instance is probably in good health - chances are your original codename has been deleted by the organization, possibly because they didn’t expect further communications.
If it does not, then there is definitely an issue with the instance itself. As mentioned previously, we have no visibility into instances, so we can’t tell which one you’re using (and you should not tell us, for anonymity-preservation’s sake). If they offer an alternative tipline mechanism that you’re comfortable using, it may make sense to reach out to them via that. If not, and you have information that you want the press to have, it might make sense to find an alternative organization with a working instance.
All the best,