Have you gotten letsencrypt certificates issued?
There are two separate steps: 1) acquiring the certificate and 2) pointing MineOS to use the constructed certificate.
1) The easiest way I've experienced working with letsencrypt is by using the stand-alone webserver to get issued the certificates, e.g.,
letsencrypt-auto then put those files here (my example):
Once the keys have been attained, you can construct the full cert chain with
cat cert.pem chain.pem > fullchain.crt
2) This cert can then be used--in conjunction with
privkey.pem--can then be plugged into
/etc/mineos.conf and restart the webserver to activate it.