SOLUTION!!
For those having the same problem, the quotes in the sample short code (“) are incorrect. They must be replaced by re-typing them. Below is what worked for me.
[simply_instagram endpoints=”users” type=”recent-media” size=”standard_resolution” display=”50″ auto_play=”true” theme=”facebook” width=”150″]
I’m sure that the developer is busy, but this plugin really needs better documentation.
P.S. Thanks to @matreece for figuring this out and posting it!