This message suggests that the certificate verification failed for the secure connection to
registry.terraform.io. In particular, it’s saying that the certificate is signed by a certification authority that is not trusted by your system’s certificate store.
In the current implementation of
registry.terraform.io the certificate is one issued to the CDN it’s currently using (Fastly) by GlobalSign CloudSSL, so I’m surprised that your system would not trust it. Is there anything unusual about how you have TLS certificates configured on your computer?
Another possibility is that your request is being intercepted by a proxy server that presents its own TLS certificate, signed by itself rather than by a trusted certification authority. In that case, Terraform would reject it because this would be the circumstances of a “man-in-the-middle” attack that could allow an attacker to send you a malicious provider instead of the one you intended to install. However, if you are accessing the registry via a local proxy server and you trust that proxy server then I think you could add that server’s CA certificate to your trusted store so that Terraform will consider it to be a valid certificate.