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

This failed message indicates that the sequence template uses variables that the prospect does not have populated on their profile. Outreach provides you with 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, you can either enter these fields into the prospect's profile or update the sequence template so it does not use these variables.