Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. https://crt.sh/?q=example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
Use whatever ACME client you choose to request a cert for that subdomain? I'm sorry, but with the information you've provided, that's really all that can be said. See: