New flow won't show Shopify coupon

I’m using the new flows and trying to set up a flow that triggers a coupon code to be sent (already created within Klaviyo under the Shopify coupon tab) after a customer subscribes to a newsletter.  The flow works without including the coupon code, so the flow is fine.  I follow the instructions to insert a text block in the email within the flow to include {% coupon_code 'NAME_OF_YOUR_COUPON' %} with the name of the coupon there, and the flow won’t allow a test email to be sent because of invalid syntax. 

Is there a different way to insert the Klaviyo Shopify coupon code into an email flow? This coupon is not used in any campaign.

Thanks