After activating YUI compressor, it says:
"It seems Java is not installed on your machine", although it does.
If I ignore the message and try to compress a .js file, it says:
"The system can not find the file specified".
Sometimes you have to start the software as Administrator to give it access to the system partition. Since Java is installed on the System partition, the software also needs exclusive access rights to call various files or modify them. There is also another possibility that your Java is not installed correctly. I mean for the version. Although in some cases, you can have any version of Java, some applications are developed to run only on a few versions.