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Since each release of OpenJFX is paired with a corresponding release of the JDK, you should make sure that you have a recent (preferably the latest) promoted build of the JDK available. For The current supported build can be downloaded Java SE 8 , these can be downloaded from the download page, or the latest early access from JDK 8 java.net page. Some make it a practice to always run against the latest promoted build, others will stick with an older build until they finally can't build OpenJFX with it anymore, and then update. Whichever method you chose, you need to have a reasonably recent version of JDK 8 installed.
The OpenJFX build requires a Java JDK 8 that does not have the JFX jar present. The build scripts check for this condition, and will refuse to continue if it is found.This jar is found in the JDK at (your JDK)/jre/lib/ext/jfrt.jar. A common practice is to have a copy the JDK for general use, and another for building, and then remembering to set you your PATH when building to the proper JDK. There are many ways to copy the JDK, and which one to use can depend on which OS you are using.
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