Thank you for reaching out to Constant Contact's API Support.
There are a few different things that can cause that error. If this is a new account make sure that your account has the physical address set under the My Settings section. Also make sure that you have verified the reply-to and from address you are using.
As for email campaign specific issues make sure that you have assigned a contact list to the email and ensure that you have set a valid send time. If all of these are set and correct can you please provide a code snippet of what you are submitting when this error occurs?