$ sudo virsh net-start default
error: Failed to start network default error: Unable to create bridge virbr0: Package not installed
Or turn on the VM using virt-manager:
Error starting domain: Requested operation is not valid: network 'default' is not active
Solution
Enable libvirtd to start with the system.
systemctl enable libvirtd
reboot
HVN at http://pymi.vn and https://www.familug.org.