Q: What does this mean? "There are no contacts associated with this Activity. Message sending is skipped"
A: The 'With' needs to have a selection like 'Buyer' or 'Seller' or whoever you plan to email. Then make sure a contact is assigned to that party type in the people tab. The email has to know who to send to and that message tells me that they are not selected. You must also make sure that the contact is not set to "Active: No". This setting is at the bottom of a contact record.