Three Things I Tried to Fix a Failing Connection From Ulysses to My Custom WordPress Site
 - I didn’t actually try this one (misdirection!) but it could be your issue. Shared hosting can block access to the /xmlrpc.phpfile. This plugin can help you get around that.
 https://wordpress.org/plugins/rename-xml-rpc/
 Be sure to also follow the installation instructions: https://wordpress.org/plugins/rename-xml-rpc/#installation
- php-xmlrpcis not installed.- sudo apt-get install php-xmlrpc. Then- sudo service apache2 restart.  Not my issue, but kept on looking.
- php-xmlnot installed.- sudo apt-get install php-xml. Then- sudo service apache2 restart. This was my issue!