Thank you for reaching out to the Constant Contact Community. It's my pleasure to assist you this weekend.
This may be happening for several reasons. There are trial limitations in place. One of them is that you may only send to 100 contacts. If you create a list with just 100 contacts on it, it will allow you to send. Please feel free to reach out should you need anything else. Thank you for choosing Constant Contact. Have a great weekend.