It seems like you might already have an SSL VirtualHost for your domain, but it is missing SSLCertificateFile/SSLCertificateKeyFile lines.
What’s this show?
apachectl -S
You may be able to manually repair the file by adding two lines to the VirtualHost:
SSLCertificateFile /etc/letsencrypt/live/www.posplus.co.za/fullchain.pem
SSLCertificateKeyFile /etc/letsencrypt/live/www.posplus.co.za/privkey.pem