I ran this command: certbot certonly --webroot --test
after running the above command, I've entered my domain name "login.saunitech.com" and then entered the webroot location on the server.
To fix these errors, please make sure that your domain name was
entered correctly and the DNS A/AAAA record(s) for that domain
contain(s) the right IP address.
Yes, exactly! In fact, the server's behavior there suggests your IIS is configured to block serving files without extensions, which will interfere with certbot --webroot.
The 1.txt and 1234.txt files return the word "sas", while the 1234 file returns a server 404 error (even though you showed in your screenshot that it's located in the same place).
I'm not familiar with IIS, but there have been threads on this forum in the past about how to configure IIS to allow it to serve files with no extension. Alternatively, you could use a client application other than Certbot with more direct IIS integration.
If 1234.txt works, but 1234 not (and if both files exist, change your folder setup to see known file extensions, not only the icons), the reason is that missing configuration.