It would be really helpful to see the specific error that the certificate authority is giving in this case. Can you see if letsencrypt.exe creates any log files that record the error message?
If not, can you try at least temporarily using a different client application?
In addition to the downloadable Windows-based clients, there are web-based clients like https://www.zerossl.com/ where you complete the steps in your web browser instead of on the command line. (This is not really preferable to running an application on the server in terms of things like automated renewal, but it might be useful for debugging purposes to figure out a more specific reason why the certificate isn't being issued.)