Possible reasons for the { “statusCode”: 404, “message”: “Resource not found” } Exception from Salesforce Workflow Outbound Message are

1. Endpoint URL is incorrect.

2. Request method is incorrect. For example, GET is used instead of POST.

3. If you are using Certificate, make sure it is not expired.

To troubleshoot or debug this “Resource not found” Exception in Salesforce Workflow Outbound Message, check whether you are able to make the Call out from tools like PostMan, SOAPUI, etc.