'authority' Uri should have at least one segment in the path (i.e. https://<host>/<path>/...) about letsencrypt-siteextension

[TWith2Sugars]

I had this problem as well, you need the ApplicationSettings set up first:

[sjkp]

@JamesReate as @TWith2Sugars mentions you have to setup the proper app setting first. That is also what the text on the first page in the site-extension tries to tell you.
I will in the next version add some more validation to avoid this problem.
The reason for the settings to be in your appsettings, are so they are available for the web job, so it can renew the certificate, otherwise you would have to manually renew the certificate, which is not the purpose of letsencrypt certificates.

[sjkp]

A side note, I could just let you add the settings from the site extensions, but I like to avoid that as it would restart your site.

[JamesReate]

Ahh, I see, I thought that part was optional or would get added automagically from the config screen.