https://gluonhq.com/products/javafx/ JAVAFX lib
Create a new User Library under Eclipse -> Window -> Preferences -> Java -> Build Path -> User Libraries -> New. and include the jars under the lib folder from JavaFX 15.
Click on Run -> Run Configurations... -> Java Application, create a new launch configuration for your project named hellofx and add these VM arguments:
--module-path \Users\Utente\Desktop\openjfx-11.0.2_windows-x64_bin-sdk\javafx-sdk-11.0.2\lib --add-modules javafx.controls,javafx.fxml