Insomnia and NodeJS: "Error: Peer certificate cannot be authenticated with given CA certificates"
There is little documentation on how Insomnia handles certificates. As long as they are normal certificates that are signed by a typical CA authority there is typically no problem. Since you also refer to your loopback address (127.0.0.1) I assume that you're also testing with a self-signed certificate.
I noticed that Insomnia uses the Mozilla list of certificate authorities. It does not use the list of your operating system.
The list is stored in a text-file in a directory like C:\Temp\insomnia_5.12.4. In my case it was for example 2017-01-18.pem. You can add your own signing authority certificate to this file.
I didn't look into how stable this file is or how it is created.
You can also workaround the certificate errors by disabling them in your settings (settings > Validate SSL Certificates).