Whenever an email doesn't send because you've included a Snippet and you don't have the corresponding value for a particular Contact, you'll know whether or not you've solved the problem based on if you still see the problem snippet highlighted in dark red in your email's Subject line or body text itself.
How to Find Emails "Not Sent" Because of Missing Snippets
1. Navigate to your Sequences page in the left main menu and click to view any individual Sequence that includes any "Not Sent" Contacts:
2. Click to view the Emails tab of your Sequence, then use the Not Sent Reason dropdown filter in the bottom-left to select Missing Snippet, and click any individual listed email to view its contents:
When the Snippet is Still Missing and Resending the Email Will Not Work
Check the light red banner at the top of the email modal to see which snippet is missing from your subject line or body text, then look for the actual missing snippet highlighted in dark red in your email's content:
When the Snippet is Fixed and You're Good to Reschedule/Resend
Simply edit the email to replace your missing snippet with replacement text or edit that Contact to fill in the value you're missing (in this example, add the first name to that Contact's First Name Field). As Long as you do not see the snippet highlighted in dark red anymore in your email's subject line or body text, the email will send even though the top banner doesn't is still there:
This email is now good to go - just click Edit Delivery Schedule to schedule the email to resend at a specific time in the future or click Force Resend to send it right away.