You mean getting a CA-issued certificate for a host name like "myfileserver", without a domain name? No, if that were allowed, you could use that globally trust

You mean getting a CA-issued certificate for a host name like "myfileserver", without a domain name? No, if that were allowed, you could use that globally trusted certificate to go play man-in-the-middle on somebody else's network which also happens to have a "myfileserver". In general, certificate authorities should only issue certificates with unambiguous subjects. Of course, if you don't need a certificate that is automatically trusted by everyone, then you can still generate a certificate yourself, without the help of a Let's Encrypt or any other CA. You would have to configure the machines on your network to trust it though.