Twig Templates - not showing when creating an email

We’ve just installed the Twig Templates plugin under v. 2.15.2. It is showing up in the left hand nav, and I’ve been able to create and test a twig template.

When I create an email and try to insert a twig template, it doesn’t appear in the drop down and doesn’t autocomplete if I try typing it into the body.

Any ideas on what I need to do to get the twig templates into an email?