how can I update my IMAC from 10.11.6 to 17

The model year of an iMac is available at Apple menu > About This Mac.

An early 2009 iMac can be upgraded to El Capitan 10.11.6 but no later version. See: How to download and install macOS - Apple Support

A late 2009 through mid 2011 iMac can be upgraded to High Sierra 10.13.6 but no later version. See: How to download and install macOS - Apple Support

A late 2012 through late 2013 iMac can be upgraded to Catalina 10.15.7 but no later. See: How to download and install macOS - Apple Support

A mid 2014 and late 2014 iMac can be upgraded to the latest version of Big Sur macOS 11 but no later. See: How to download and install macOS - Apple Support

A late 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. See: How to download and install macOS - Apple Support

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. See: How to download and install macOS - Apple Support  

A 2019 or later iMac can be upgraded to the current version of Sonoma macOS 14. See:How to upgrade to macOS Sonoma - Apple Support

There is no such thing as macOS 17, yet. Sonoma (macOS 14) is the latest version. The Related article is about iOS 17 (the latest operating system for iPhones), so it seems that you have the Mac and iPhone OSes mixed up.

None of the 21.5" iMacs that are old enough to run El Capitan (10.11.6) are new enough to run Sonoma (14).

The latest version of macOS that you can run might be as low as High Sierra (10.13.6) or as recent as Monterey (12), depending on your hardware model year. What does it say about the model year in  > About This Mac ?

I have a 2011 iMac, just tried installing high sierra now it’s stuck in a loop “resources cannot be found” and it won’t log on just keeps coming up “restart” been on to support chat, they’ve booked me in a Genius Bar, as they can’t fix it, any ideas?

I would expect macOS 17 to be released in the Autumn of 2026.

Something like that. 😎