You need to install Java. Then open the FutureNotepad.jar file with Java. Did that solve your problem?
Wait, I think I know why you getting this error! When you run a Java version that is below 11.0.12 (the version I used to develop future notepad), it won't work! You can get the latest version from here: https://www.oracle.com/java/technologies/javase-jdk16-downloads.html. Did that finally solve your error?