What does the failed message 'missing template variable' mean?

This failed message indicates that the prospect does not have all of the fields you used as variables in your template filled out in their profile. Outreach gives you a list of the missing fields in this error message. 

For example:

This prospect was missing the nickname, company, and work phone fields.

To fix this, enter these fields into the prospect's profile or change your template so it does not use these variables.