Ok, this is baffling. I am trying to use a simple line

Ok, this is baffling. I am trying to use a simple line of shortcode to show one category of posts on a page. Nothing else on the page but a little text as I wanted to test the posts came through correctly first.

This is all I have put to get this to work:
---
Sponsor a senior

 

Paypal button
[display-posts category="seniors"]
---

But, it displays the Shortcode instead of creating the object to show the "seniors" blog posts. (see the image attached)

URL is http://coloradosaintbernardrescue.org/sponsor-a-senior/