I have OS X 10.8.5, and I want to upgrade to High Sierra. When trying to download, I get: "This version of macOS 10.13.5 cannot be installed on this computer." What to do?
MacBook Pro, OS X Mountain Lion (10.8.5)
That model can not be upgraded beyond 10.11.6.
Which model of MacBook Pro do you have? Is it a Late 2009 or newer model?
Check to make sure your applications are compatible.
Application Compatibility
El Capitan 10.11 Compatibility information
Also check to make sure there is a compatible driver for your printer.
Do a backup before installing, preferable 2 backups on 2 different drives.
Mid 2009
High Sierra from Mountain Lion 10.8.5
