Loopback request to your site failed

How do I get loopback request to finalise please? I recently installed Health check plug in to show me exactly what is wrong with the LB request but got very little information. I only got this message:
“The loopback request to your site failed, this means features relying on them are not currently working as expected.

Error encountered: (0) cURL error 56: Failure when receiving data from the peer.”