@mv15 here is some supporting documentation: https://docs.certifytheweb.com/docs/http-validation.html
Your website is specifically denying access to the file. This is usually because the parent application has authentication switched on.
Can you look at your web.config that should be in the /.well-known/acme-challenge of your website? It should have an entry like this, to allow any user to access the folder:
<system.web>
<authorization>
<allow users="*" />
</authorization>
</system.web>
You may need to add an exception to your parent application web.config like this (the syntax might not be exact but you’ll see the general idea):
<location path=".well-known/acme-challenge">
<system.web>
<authorization>
<allow users="*"/>
</authorization>
</system.web>
</location>
</configuration>```