OpenTAXII: Cannot poll feeds from splunk

I am trying to poll data from splunk and here are my configure: URL: http://<IP>:9000/services/test_all_poll POST arguments: collection="test-all-collection" taxii_username="username" taxii_password="password"

I am sure that the username and the password are correct, but here is what I got back {"exception": "Traceback (most recent call last):\n File \"/usr/lib64/python2.7/site-packages/flask/app.py\", line 1813, in full_dispatch_request\n rv = self.dispatch_request()\n File \"/usr/lib64/python2.7/site-packages/flask/app.py\", line 1799, in dispatch_request\n return self.view_functions[rule.endpoint](**req.view_args)\n File \"/usr/lib/python2.7/site-packages/opentaxii/middleware.py\", line 76, in wrapper\n raise UnauthorizedException()\nUnauthorizedException: UNAUTHORIZED", "level": "warning", "timestamp": "2018-06-12T05:58:33.217832Z", "logger": "opentaxii.middleware", "event": "Status exception"}

Does anyone know why this would happen? And where can I check the log record? Thanks.