Apple increased the minimum system requirement for running the High Sierra 
installer from 10.7.5 to 10.8, and also increased the estimated free space 
you’ll need to install it. 

https://www.apple.com/macos/how-to-upgrade/
General Requirements
OS X 10.8 or later
2GB of memory
14.3GB of available storage to perform upgrade*.                                
    





Sent from Ronni's iPhone 7 Plus
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