On Linux (and generally, on Unix systems using the X11 window system), the DISPLAY environment variable tells how to connect to the X server, which manages the graphical display. Apparently something tried to open a window. I don't know anything about netbeans, but I see two possibilites:

• Either, you didn't expect a window. In that case, you probably installed something wrong, or run something different than you expected. Or maybe you have to add some option to disable the GUI.

• Or you did expect a window. In that case, the problem is most probably not related to netbeans, and the solution is dependent on how exactly you start the software (locally from a terminal, remotely through SSH, or in some other way).

If you're starting locally from the terminal, it might be sufficient to first issue the command export DISPLAY=:0.0 before starting netbeans. If you're doing it remotely, you definitely do not want to do that.

After executing export DISPLAY=:0.0, I got the following error which running netbeans: https://paste.ubuntu.com/p/JmYv8Ng67T/

I thought I am getting the error cause netbeans couldn't find JDK. So I had executed it again

netbeans_jdkhome='/usr/bin/javac'

although I still get the same error.

I notice that your Java installation line contains the word “headless”. “Headless” basically means “without screen”. Therefore I'd guess that the remaining problem is that you installed the wrong version of jdk. From a bit of internet search, I suspect that you'll get what you want with sudo apt install default-jdk (no guarantee, though).

https://software.codidact.com/posts/284357