Windows 10 Bash "Cannot connect to display!"

You need to install an "X server" to provide that graphical interface which is yet not there on a typical Ubuntu subsystem on windows 10.

If you install an X server application on your Windows desktop and change a setting in the Bash shell, applications will send their graphical output to the X server application and they’ll appear on your Windows desktop.

You can install Xming a good X server for Windows. Just accept the default settings. It will then automatically launch and run in your system tray, waiting for you to run graphical programs.

After installation you’ll need to set the DISPLAY environment variable to point at the X server already running on your system.

First run the following command in the Bash environment to set the DISPLAY variable:

export DISPLAY=:0

and then run your graphical application. In this case

suplane | suximage title="My first test" &

^{Some part is taken from howtogeek}