thanks for the reply
not sure if that's the problem tho?
I have very limited knowledge on ssl so I could be wrong but the same error occurs when I type
certbot certonly and type either 1(standalone) or 2(webroot)
I've tried copying .pem files in
/etc/letsencrypt/archive/hansungsmb.co.kr/ to
/etc/pki/ca-trust/source/anchors/
and ran update-ca-trust but no difference yet 