First, confirm that your vendor credentials are correct. The vendor generates these credentials, so you may need to contact them for confirmation.
If your credentials are correct, ensure that you've entered all of the required information for configuring this vendor. You can check the COUNTER Registry to see what is required for each vendor. Some vendors require an API key to be entered, for example.
If you've confirmed that you are using the correct credentials, contact OCLC Support with your OCLC Symbol and the name of the vendor you are configuring.