Verify that you are typing in the right username and password. To confirm that you are entering the correct credentials, follow the steps below:
  1. Go to Setup > select Site Options
  2. Select the Open API Configuration tab
  3. Click "Edit server API configuration"
  4. Verify that your credentials are in this list. If not, then add them
If you are using the right credentials but still can't log in, make sure you aren't using too many characters in your password and avoid using uncommon special characters.