If the following properties (of a javaFX maven based project) : "Java platform" and/or "Source/Binary format" are set to anything higher than "JDK 1.8", then "Clean and Build" (as well as "Run") operation would fail.
Different combination o these parameters would produce different errors.
To reproduce, simply create a new JavaFX Maven Project: New Project -> Maven: Java FX Application. And then try to Clean and Build / or Run.
The most typical error is
Cannot run program "/Library/Java/JavaVirtualMachines/jdk-10.0.1.jdk/Contents/Home/bin/javafxpackager"
(Note, as of 1.8 the javafxpackager does nto exist anymore)