X11 forwarding request failed on channel 0

I cannot open X11 when I connect to OS X Sierra from OS X El Capitan (or from any older OS Xs).

Once I "ssh -X" or "ssh -Y" into Sierra I get

"X11 forwarding request failed on channel 0"

If I type "echo $DISPLAY" I get an empty line.

I tried to follow the suggestions from

x11 Forwarding

but that discussion applied to Yosemite. I still tried to modify the Yosemite file /etc/sshd_config which I think is now the Sierra file /etc/ssh/sshd_config and add the lines

X11Forwarding yes

X11DisplayOffset 10

X11UseLocalhost no

although I noticed the Sierra file has a bit different names, e.g., ForwardX11 instead of X11Forwarding.

Adding the above three lines did not solve the problem. Interestingly, when I tried to restart the ssh service following the suggestions from the above discussion:

cd /System/Library/LaunchDaemons

launchctl unload ssh.plist

launchctl load ssh.plist

I noticed the following: after typing "launchctl unload ssh.plist" I got

"Could not find specified service"

and after typing "launchctl load ssh.plist" I got

"Service is disabled"

I then rebooted the computer but that did not help either.

I would appreciate any input.