Xampp, windows, this is the readiness check when going from 2.1.5 CE to 2.1.6 CE
Your current PHP memory limit is 6G. Magento 2 requires it to be set to 756M or more. As a user with root privileges, edit your php.ini file to increase memory_limit. (The command php --ini tells you where it is located.) After that, restart your web server and try again.
Last time I checked, 6G was a little bit higher than 756M. Such a sloppy error message would be amusing were it not on a business critical system. Suffice to say, we'll stick with 2.1.5 for the moment.