Here are a few steps you can take to troubleshoot and resolve the issue:

1. Make sure you have the correct URL structure for the endpoint. The format should generally be:
   https://your-osticket-domain/api/tickets.json
2. Ensure that you're providing the required authentication credentials (e.g., API key, OAuth token, username/password) in the request headers or parameters. Without proper authentication, the API might reject your request.
3. Check that you're using the correct HTTP method for the request. If the API documentation specifies a specific method (e.g., GET, POST), make sure you're using it correctly.
4. Double-check that you're passing the ticketNumber parameter correctly in the request. It should be included in the URL as a query parameter.
5. Confirm that you're using the correct version of the osTicket API. B2B API meaning - (https://www[dot]globaldatabase[dot]com/why-use-b2b-data-providers-apis-when-building-your-app), endpoints and structures can change between different versions.
6. Refer to the official osTicket API documentation for the exact endpoint details, required headers, parameters, and any specific integration guidelines.
7. If the API returns an error response, examine the response body or headers for more information about the error. This might give you insights into what's going wrong.
8. If the ticket number contains special characters, ensure that it's properly URL-encoded in the request URL.
9. Sometimes the error might be related to network connectivity or server issues. Ensure that your server or development environment can reach the osTicket API server.