All Usenet downloads must be downloaded using an .nzb file. This contains all of the content necessary to download the completed file you are looking for. The .nzb may be uploaded directly to TorBox, or you may provide a URL that TorBox can "grab" and get the .nzb file from. This requires the URL to point to a direct download containing the .nzb file.
If using an 3rd party integration, make sure that the URLs are accessible.
If using the Search Engine feature (BYOI), then you need to make sure that the search engines you add are publicly accessible from any IP.
We recommend protecting your instance using authentication in this case. If you don't like the idea of opening up your network, this feature is not for you. You may protect using Cloudflare, hosting the search engine on a VPS, add authentication, or port forward.
If using the TorBox Voyager Search Engine (without using the bring your own indexer feature), this error is abnormal, and you should report it.