Hello @misterprotocol, welcome to the Let's Encrypt community.
FTP is not in the set of Challenge Types - Let's Encrypt
There is
- HTTP-01 challenge challenge can only be done on port 80.
- DNS-01 challenge your client will create a TXT record derived from that token and your account key, and put that record at
_acme-challenge.<YOUR_DOMAIN>
. - TLS-ALPN-01 it is performed via TLS on port 443.