Cannot find the class file for java.lang.Object in JavaFX

Question

I'm following this JavaFX tutorial in Eclipse, but when I try building my project, I receive an error:

The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project JavaFXProject Unknown Java Problem

I have tried to fix the build path, but it's not working. Here's a list of the configuration contents installed in Eclipse:

And here is the external JAR file already added to the library:

Just found that the problem is that libraries are missing from the JAR file. See the screenshot below. What could be the solution now?