404 Error on print link

I’m getting a 404 error when clicking the link on a post.

https://blog.mysite.com/1174/post-title

I click the “print this post” link, which is

https://blog.mysite.com/1174/post-title/print/

and I get a 404 page not found error.

It’s a modified twentysixteen theme. I added the code to content-single.php.