Some of the errors I ran into

• DISPLAY is not set
• Failed to allocate internet-domain X11 display socket
• X11 forwarding request failed on channel 0

1) Make sure you have the X11forwarding enabled.

Server: /etc/ssh/sshd_config

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X11Forwarding yes
X11DisplayOffset 10
AllowTcpForwarding yes`

Client: /etc/ssh/ssh_config

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ForwardX11 yes
ForwardX11Trusted yes

2) Enable lightdm to listen on TCP sockets

Modify and add line below to: /etc/lightdm/lightdm.conf

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xserver-allow-tcp=true

3) If you have disabled IPv6, one more update to sshd_config is needed
Modify and add line below to: /etc/ssh/sshd_config

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AddressFamily inet

4) Restart lightdm and sshd – or – Reboot