Because of JavaFX's history and because it must be loaded from the module path and not the class path, things are a bit odd. I would recommend the following.
1. Install the unofficial OpenJFX features from the following update site. The standard, swing, and SWT features should be sufficient. [These features aren't typical eclipse plugins. They just provide the JavaFX modules so that they can be loaded. They don't export packages and aren't required JavaFX jmods were included when building the target JRE]https://downloads.efxclipse.bestsolution.at/p2-repos/openjfx-16
2. Verify that e(fx)clipse is configured as a framework extension in your eclipse.ini. The feature for e(fx)clipse may need to be installed but it may be already installed as part of the Eclipse Modeling Tools package. Ensure the following is included in your eclipse.ini: -Dosgi.framework.extensions=org.eclipse.fx.osgi
Principal Research Scientist
Rotorcraft Systems Engineering and Simulation Center
University of Alabama in Huntsville