Hi @cg51,
That means that your IIS Server should be able to serve files without extension, so your web server not only needs to be able to serve testfile.html but testfile also because the challenge used to verify your domain uses a file without extension.
Take a look to this post How To: Get LetsEncrypt working with IIS manually - #4 by JSpraul if you need to know how to configure it.
Cheers,
sahsanu