Check the following to ensure your email test can be sent:
Ensure the custom values being used are valid. Be sure that custom values have proper brackets {{ }} (not just 1 {} or more than 2 {{{}}}). For example, {{contact.name}} or {{contact.city}}.
Check the email address to ensure it follows the proper format such as test@emaildemo.com
NOTE: Check out our other article "How to Send a Test Email and SMS" for our best practice recommendations for testing emails in a campaign.