After installing JDK, unable to run .jar

From your log its clear that you'r using JDK-12, and it seems that your jar is developed with JDK-8 where this version of JDK contains the JavaFx library.

But Oracle is removing JavaFX from the JDK-11, so it is no longer present in the JDK-11+, instead you have two solutions :

1. use JDK-8 which contains this JavaFx,
2. or include the JavaFx package or dependency in your project check the link