By default, Cloud VPS comes with preinstalled CentOS6, so you need to recreate it with CentOS7. Please look here for detailed instructions.

After recreating the server, you will have to disable firewall settings with this command;

# systemctl stop iptables

And then, confirm if you are able to connect to Plesk.

* Firewall will be active after VPS reboot, so please do not forge to set firewalls and  [SAVE]  it via Plesk after disabled firewall with above command.

Please see here for details about accessing Plesk for the first time.