cannot resolve symbol javafx.application in IntelliJ Idea IDE

As indicated here, JavaFX is no longer included in openjdk.

So check, if you have <Java SDK root>/jre/lib/ext/jfxrt.jar on your classpath under Project Structure -> SDKs -> 1.x -> Classpath? If not, that could be why. Try adding it and see if that fixes your issue, e.g. on Ubuntu, install then openjfx package with sudo apt-get install openjfx.

This should be your perfect solution. Try and enjoy.If some command does not work properly that means if you get any error, try to solve it yourself. I have given you the main thing you need. If your application is in a different location, or your system architecture is different, solve it yourself. It's very easy to do it. Just follow my given solution.

Step 0:

sudo apt-get install openjdk-8-jre

Step: 1

sudo apt-get install openjfx

Step 2:

sudo cp /usr/share/java/openjfx/jre/lib/ext/* /usr/lib/jvm/java-1.8.0-openjdk-amd64/lib

Step 3:

sudo cp /usr/share/java/openjfx/lib/* /usr/lib/jvm/java-1.8.0-openjdk-amd64/lib

Step 4:

sudo chmod 777 -R /usr/lib/jvm/java-1.8.0-openjdk-amd64

now open a new project or rebuild your project.Good luck.

You can use the one that comes with IntelliJ: <intellij>/jre64/lib/ext/jfxrt.jar.