"CERT_AUTH_REQUIRED" is returned if the cert login request is made but the presence of the certificate in the request isn't identified.
Please check that the certificate is uploaded to your Betfair account.
Please also check your login code correctly references the directory in which your certificate is being stored.
Please can you also try using the curl request example below and let us know if you receive a valid response.
curl -q -k --cert client-
.crt --key client-
-d "username=testuser&password=testpassword" -H "X-Application: curlCommandLineTest"