In IntelliJ Idea,
Check the following things are configured properly,
Step 1:
File -> Setting -> Plugins -> search javafx and make sure its enabled.
Step 2: Project Structure (Ctrl+Shift+Alt+s)
Platform Settings -> SDKs -> 1.8 -> Make sure Classpath should have "jre\lib\ext\jfxrt.jar"
Step 3:
Project Settings -> Project -> Project SDK - should be selected 1.8
Project Settings -> Project -> Project language level - configured as 8
Ubuntu: If not found jfxrt.jar in your SDKs then install sudo apt-get install openjfx