My old Imac(OSX EL Capitan 10.11.6) does not recognise my iphone 14

Your iMac is too old to regognized your new phone. What is your iMac's model? You can see that via the apple logo in menu bar>about this Mac.

You see your Macs model name and year there, please report it so we can see if your Mac can upgrade any further then el cap

iPhone 14 - Technical Specifications states that you can do syncing with iTunes 12.8 or later, running on El Capitain 10.11.6. The iPhone 14 originally shipped with iOS 16.0.

iPhone 15 Plus - Technical Specifications says that you need iTunes 12.9 or later, running on High Sierra 10.13. The iPhone 15 Plus originally shipped with iOS 17.0.

The OP's tag line says that he is running iOS 17.0 on his iPhone 14. Perhaps one side effect of the upgrade was to raise the requirement for the Mac or PC used to manage the iPhone … and the specifications for the new iPhones (which originally shipped with iOS 17.0) tell us what the new requirement is.

For early 2009 iMacs, El Captain is the end of the line.