The merchant login ID or password is invalid or the account is inactive
If you are using Authorize.net as a gateway then usually this means the account set up is not finalized or the merchant account is not set up completely. Usually a call to authorize.net or signing in to your Authorize.net account will fix it.
Contact Info for Authorize.net:
Toll-Free Phone: 877-447-3938
Authorize.net Support Hours:
M–F: 5 AM – 5PM PST
(closed major holidays)