How to have qubes.ConnectTCP in place properly

As a reference from this topic, there is only one solution.

In order to fix error 126, the first thing is to edit /etc/qubes-rpc/policy/qubes.ConnectTCP as this following for one line only:

$anyvm $anyvm allow

This way you don’t have any errors.

Another thing that in order to update is this following command in dom0 terminal:

sudo qubes-dom0-update --action=reinstall binutils

This will update your commands because you are adding something to your machines.

Vola! you are done there, but don’t add anything to qubes.ConnectTCP file otherwise all vms cannot update.

As per documentation, the $ character has been deprecated in favor of @:

(Note: the $ character is deprecated in qrexec keywords – please use @ instead (e.g. @anyvm). For more information, see the bulletin here.)

\ RPC policies | Qubes OS
\ qubes-secpack/qsb-038-2018.txt at master · QubesOS/qubes-secpack · GitHub

Furthermore, you should really consider the possible security implications of allowing unregulated TCP connections between any vm.
The solution you proposed shouldn’t be the answer to your issue, as I’m able to update everything just fine without that insecure policy in place.