In order to javafx work, you need to either download and import javafx sdk to your project or specify dependency, if you are using build tool like Maven or Gradle.
Moreover you need to add required modules (in this case javafx's .jars) either by using module-info.java and specifying required modules there, or adding modules to VM arguments - https://openjfx.io/openjfx-docs/#install-javafx.
Bonus:
Note that javafx was present in JDK up to version 10. Since version 11, they removed it. That is why you need to import javafx somehow to your project.
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