How to install Let's Encrypt SSL certificate on a subdomain?

Question

Digital certificates

Hello dear! There is a problem. I can't put a free Let's Encrypt certificate in any way: I
need to put it on a subdomain.
The main domain is located on server A with ip 1 (without a certificate) The
subdomain is located on server B with ip 2 (without a certificate)
they have common DNS.
I install it on a subdomain on server B with ip 2, as a result, the error is:

IMPORTANT NOTES:
- We were unable to install your certificate, however, we
successfully restored your server to its prior configuration.
- Congratulations! Your certificate and chain have been saved at
/etc/letsencrypt/live/forum.asdasd.ru/fullchain.pem. Your cert
will expire on 2016-10-27. To obtain a new or tweaked version of
this certificate in the future, simply run letsencrypt-auto again
with the "certonly" option. To non-interactively renew *all* of
your certificates, run "letsencrypt-auto renew"
- If you lose your account credentials, you can recover through
e-mails sent to dasdasd.com.
- Your account credentials have been saved in your Certbot
configuration directory at /etc/letsencrypt. You should make a
secure backup of this folder now. This configuration directory will
also contain certificates and private keys obtained by Certbot so
making regular backups of this folder is ideal.

I suppose you just need to transfer the main domain to server B with ip 2?
Or what's the problem?

Answer 1

Tried to reinstall, got this error:

Error while running apache2ctl configtest.
Action 'configtest' failed.
The Apache error log may have more information.

apache2: Syntax error on line 219 of /etc/apache2/apache2.conf: Syntax error on line 18 of /etc/apache2/sites-enabled/forum.site.ru-le-ssl.conf: Expected </VirtualHost> but saw </VirtualHost></IfModule>


IMPORTANT NOTES:
- We were unable to install your certificate, however, we
   successfully restored your server to its prior configuration.
[email protected]:/opt/letsencrypt#
]