I encountered the problem just as this person (https://stackoverflow.com/questions/54291958/javafx-11-illegalaccesserror-when-creating-label). Thank you for the nice pose, I finally resolve the problem.

Using JavaFX, except to designate Library, you also should add module information. Check the official guide.

JavaFX and IntelliJ / Non-moduler from IDE.

https://openjfx.io/openjfx-docs/