Hello @rwesterman, welcome to the Let's Encrypt community. 
Let’s Encrypt offers Domain Validation (DV) certificates.
Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Thus you need to own and have control over the Domain Name (or have a subdomain under an existing domain name, for example pointed to your server by your employer or school) you wish to obtain a certificate for, from an ICANN Accredited Registrar.
Please read Certificates for localhost - Let's Encrypt
Here are some link to within the forum that are worth reading