Error: “Cannot find target link”

I’m trying to set-up webmentions. When I try to manually send a webmention from /wp-json/webmention/1.0/endpoint, I get an error: “Cannot find target link.” Brid.gy also gives an error: “Propagate task failed”

I turned on debug mode and got this error: “Notice: Undefined variable: response in /home3/restlew6/public_html/wp-content/plugins/webmention/includes/class-webmention-request.php on line 229”

I am copying the target url right from a post permalink, so the page definitely exists. As far as I can tell, I have webmentions activated and comments and pingbacks enabled for the post. Am I missing something else that needs to be enabled?

The page I need help with: [log in to see the link]