Is it possible to use a LetsEncrypt cert on an internal facing Windows site that will not resolve outside of our organization? If so, what tool should I use? I tried the GUI tool Certify but it will only work if external resolution is possible.
Does it have a domain name that you have control over outside of your organization ? if so you could potentially get a certificate using a DNS challenge.
If not, and it hasn’t got a public domain name associated with it, then you can’t obtain a certificate.
Others may know better - I’m more of a Linux guru. There are windows clients at https://letsencrypt.org/docs/client-options/ at least some of which include the DNS challenge such as ACMESharp