Why am I receiving the login error ACCOUNT_PENDING_PASSWORD_CHANGE?

The error indicates that you Betfair account has been locked. 

Once you've received this error you must undergo password recovery to reactivate via https://identitysso.betfair.com/view/recoverpassword

Once you've update your password via the above process, you should be able to login to your Betfair account successfully.

Please note:  Before updating your password please ensure that you stop any existing automated log-in process that runs using your old password.  This can cause an infinite loop of INVALID_PASSWORD attempts and unless stopped will result in your account being locked out again due to too many incorrect password attempts.