Failed to register the CT: PRL_ERR_VZCTL_OPERATION_FAILED (Details: Creating OS template cache for centos-8-x86_64 template
Can’t open /proc/sys/kernel/virt_osrelease: No such file or directory
Error: /usr/share/vzyum/bin/yum failed, exitcode=1
VE_PRIVATE is not set
Creation of Container private area failed

Make sure you are using vz7 kernel:
uname -a | grep vz7
Linux CentOS-78-64-minimal 3.10.0-1127.8.2.vz7.151.14 #1 SMP Tue Jun 9 12:58:54 MSK 2020 x86_64 x86_64 x86_64 GNU/Linux