Sprout Email not triggering from Sprout Form
Variables in Craft are case sensitive.  You'll run into the same error if you try to output the Craft {{ siteName }} global as {{ SITENAME }}.

Updating the casing to {contactFullName} or {{ object.contactFullName }} should get things working for you.



An extra note regarding the comments above. Luke is onto the right answer in the comments, however, one more distinction needs to be made. There are multiple variables available to your Sprout Email templates. To quote the docs linked in Luke's comment: 


  In your notification templates you can use any Twig code that you
  would use elsewhere in your templates. Additionally, Sprout Email
  Notifications give you access to the Notification Email Object (entry)
  and any Dynamic Object provided by the event (object)


Your variable contactFullName is not a field on your Notification Email Entry, it's a field on your Sprout Forms Submission Entry.  It's correct that you would get an error trying to refer to it as {entry.contactFullName} because 1) the Notification Email doesn't have that field and, 2) the shorthand syntax is going to add the object variable behind the scenes, so the variable would be seen as {{ object.entry.contactFullName }}.