There is not enough disk space available to install the product. however i have 16gb free and i try to install xnote whixh is 6gb

That 6GB application will expand to 9 - 12 GB once uncompressed and installed. The system may need as much temporary space while installing it, so your 16GB is unusable. Free up some excess storage in your home directory, or fully uninstall some third-party applications, or games.

Let me reiterate what has been said already, and say this: 16GB is not nearly enough free space for normal operation of the system, even if you are not trying to do an update.

There's a sort of general rule of thumb that you need to keep between 15-20Gbs of free space on your Mac, this is to allow the system to do its housekeeping, the system is constantly writing and rewriting files and moving them around the disk, so this is why you are getting the warning to free up space.

Files that tend to take up the most space are Movies, Images and Music, so if you can offload some of them that are not needed on a daily basis move them onto an external drive, and delete them from your Mac. If you continue to fill up your drive then your Mac could lock up, which may need a clean install which would mean you could lose your important data.