When I try to install a new virtual machine in virt-manager Linux Mint virtualization, I get an error message error no active connection. In this situation, I’ll just go to Terminal and then paste this command to run and fix it.

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$ sudo chown $USER:$USER /var/run/libvirt/libvirt-sock

This is a solution how to copy and paste from the host to a KVM guest, just execute this command, after done then you can reboot.

$ sudo apt install spice-vdagent

After running this command all was fine. Good luck!