Please make sure Whonix-Gateway (commonly called sys-whonix) is running.

Check your _dom0_ settings in the /etc/qubes/policy.d/ folder.

To see if it is fixed, try running in Whonix Template:

sudo systemctl restart qubes-whonix-torified-updates-proxy-check

Then try to update / use apt-get again.

For more help on this subject see:
https://www.whonix.org/wiki/Qubes/UpdatesProxy

If this warning message is transient, it can be safely ignored.
