OpenVZ is not supported on Ubuntu 18.04 as OpenVZ project is discontinued and is now replaced by Virtuozzo 7 which only supports CentOS. The last version of Ubuntu to support OpenVZ was Ubuntu 14.04 LTS.
For virtualization in Ubuntu 18.04, you can use alternatives to OpenVZ such as:
1. Docker: A platform service that delivers software in packages called containers.
1. LXC: LXC is a Linux system-level virtualization method for running multiple isolated Linux systems on a single host.
1. KVM: Kernel-based Virtual Machine is a full virtualization solution for Linux on x86 hardware containing virtualization extensions.
Note: It’s always a good idea to use supported software, it will be much more secure and will generally work better.
